Mechanical component and rolling bearing

ABSTRACT

An outer ring, an inner ring and a ball serving as mechanical components configuring a deep groove ball bearing are formed of steel containing at least 0.77 mass % and not more than 0.85 mass % of carbon, at least 0.01 mass % and not more than 0.25 mass % of silicon, at least 0.01 mass % and not more than 0.35 mass % of manganese, at least 0.01 mass % and not more than 0.15 mass % of nickel, at least 3.75 mass % and not more than 4.25 mass % of chromium, at least 4 mass % and not more than 4.5 mass % of molybdenum and at least 0.9 mass % and not more than 1.1 mass % of vanadium with a remainder consisting of iron and impurity, and have raceway/rolling contact surfaces, the surface being included in a region having a nitrogen enriched layer having a nitrogen concentration of at least 0.05 mass %, the nitrogen enriched layer having a carbon concentration and the nitrogen concentration, in total, of at least 0.82 mass % and not more than 1.9 mass %.

TECHNICAL FIELD

The present invention relates generally to mechanical components androlling bearings, and particularly to mechanical components that areformed of steel containing at least 3.75 mass % of chromium and have asurface layer portion with a nitrogen enriched layer, and rollingbearings including ceramic rolling elements.

BACKGROUND ART

In order to obtain a mechanical component of steel having a surfacelayer portion enhanced in strength, a treatment is sometimes performedto provide the surface layer portion with a layer having a highernitrogen concentration than the remaining region, i.e., a nitrogenenriched layer. For example, a nitriding treatment is performed. Aconventional nitriding method for steel is representatively a gas softnitriding treatment in which steel is heated in an atmosphere containingammonia or a similar gas serving as a source of nitrogen to cause thenitrogen to penetrate into a surface layer portion of the steel.However, when steel containing chromium in a large amount such as atleast 3.75 mass %, for example, is used to produce a mechanicalcomponent, the mechanical component has a surface layer portion havingchemically stable oxide film. As such, when the mechanical componentformed of steel containing chromium in the large amount undergoes thegas soft nitriding treatment the surface layer portion is not penetratedby nitrogen and the nitrogen enriched layer is not formed.

To address this, a plasma nitriding process is proposed as follows: anobject of steel to be treated is placed in a vacuumed furnace and a gascontaining a gas serving as a source of nitrogen is introduced into thefurnace, and between the object and a member, such as a wall of thefurnace, disposed to face the object a difference in potential is causedto cause glow discharge to cause nitrogen to penetrate into a surfacelayer portion of the steel configuring the object (see Japanese PatentLaying-open No. 2-57675 (Patent Document 1) for example). The plasmanitriding process is controlled for example as based on a spectralanalysis of glow discharge, a density of a current flowing in theobject, or the like, as proposed in Japanese Patent Laying-Open No.7-118826 (Patent Document 2) and Japanese Patent Laying-Open No. 9-3646(Patent Document 3). This allows a mechanical component formed of steelcontaining at least 3.75 mass % of chromium to have a surface layerportion provided with a nitrogen enriched layer.

Following the recent improvement in performance and efficiency of amechanical device employing a rolling bearing, high durability in asevere environment tends to be required to the rolling bearing. Morespecifically, a rolling bearing employed in a contaminated environmentpenetrated by hard foreign matter may be damaged in an early stage (inan operating time shorter than a calculated life of the bearing) due togripping of the foreign matter. Furthermore, a rolling bearing rotatingat high speed may have smearing even if the bearing is under relativelysmall load. Furthermore, a rolling bearing used in an insufficientlylubricated environment may cause seizure. When the rolling bearing isused in a high-temperature environment of a temperature exceeding 200°C., for example, hardness of components (bearing components)constituting the rolling bearing may be reduced, to reduce thedurability of the rolling bearing.

When the bearing components are made of steel, strength at a hightemperature can be improved for improving the durability of the rollingbearing in the high-temperature environment by adding at least 3.75 mass% of chromium to the steel thereby improving tempering softeningresistance of the steel. In order to improve the durability in thecontaminated environment, a treatment of forming nitrogen-enrichedlayers having higher nitrogen concentrations than the remaining regionson surface layer portions of the bearing components by performingnitriding, for example, can be employed.

In bearing components made of steel having a high chromium content of atleast 3.75 mass %, for example, chemically stable oxide films are formedon the surface layer portions. When ordinary nitriding is performed onthese components, therefore, nitrogen does not penetrate into thesurface layer portions thereof, and no nitrogen-enriched layers areformed. In relation to this, there is proposed a countermeasure offorming nitrogen-enriched layers by performing a plasma nitridingprocess, as described above.

In order to improve seizure resistance, there is proposed acountermeasure of dipping balls serving as the bearing components in anorganic phosphorus compound for forming reaction layers on the surfacesthereof (refer to Japanese Patent Laying-Open No. 9-133130 (PatentDocument 4), for example).

Patent Document 1: Japanese Patent Laying-open No. 2-57675

Patent Document 2: Japanese Patent Laying-open No. 7-118826

Patent Document 3: Japanese Patent Laying-open No. 9-3646

Patent Document 4: Japanese Patent Laying-open No. 9-133130

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

Even if the nitrogen-enriched layers are formed on the surface layerportions of the mechanical component (including bearing components) madeof the steel containing at least 3.75 mass % of chromium, as describedabove, however, the characteristics of the mechanical components may notbe sufficiently improved. In other words, flaking or fracture may becaused in an early stage when stress is repetitively applied to theaforementioned mechanical components (reduction in fatigue resistance).Further, breakage may be easily caused when impactive stress is appliedto the aforementioned mechanical components (reduction in toughness).Thus, sufficient characteristics may not necessarily be attainedparticularly in fatigue resistance and toughness when merelynitrogen-enriched layers are formed on the mechanical components made ofthe steel containing at least 3.75 mass % of chromium, although thesurface layer portions thereof are improved in hardness.

The environment in which the rolling bearing is used is becomingincreasingly severe. To a rolling bearing used for a jet engine of anaircraft or the like, for example, not only suppression of reduction inhardness of bearing components in a high-temperature environment andimprovement in durability in a contaminated environment and smearingresistance, but also improvement of seizure resistance in a case wherelubrication is temporarily stopped (improvement of the so-called dry-nmperformance) are required. Therefore, conventional countermeasuresincluding those disclosed in the aforementioned Patent Documents 1 to 4cannot necessarily be regarded as sufficient.

Accordingly, an object of the present invention is to provide amechanical component that is formed of steel containing at least 3.75mass % of chromium, and also has a surface layer portion having anitrogen enriched layer, and also ensures sufficient fatigue resistanceand toughness. Another object of the present invention is to provide arolling bearing capable of attaining not only suppression of reductionin hardness of bearing components in a high-temperature environment andimprovement in durability in a contaminated environment and smearingresistance, but also improvement of dry-nm performance.

Means for Solving the Problems

A mechanical component according to one aspect of the present inventionis constituted of steel containing at least 0.77 mass % and not morethan 0.85 mass % of carbon, at least 0.01 mass % and not more than 0.25mass % of silicon, at least 0.01 mass % and not more than 0.35 mass % ofmanganese, at least 0.01 mass % and not more than 0.15 mass % of nickel,at least 3.75 mass % and not more than 4.25 mass % of chromium, at least4 mass % and not more than 4.5 mass % of molybdenum and at least 0.9mass % and not more than 1.1 mass % of vanadium with a remainderconsisting of iron and impurity. The mechanical component is providedwith a nitrogen-enriched layer having a nitrogen concentration of atleast 0.05 mass % formed on a region including a surface. The total of acarbon concentration and the nitrogen concentration in thenitrogen-enriched layer is at least 0.82 mass % and not more than 1.9mass %.

The inventors have conducted detailed studies as to the reason whyfatigue resistance and toughness are reduced when a nitrogen-enrichedlayer is formed on a mechanical component made of steel containing atleast 3.75 mass % of chromium. As a result, it has been recognized thatthe fatigue resistance and the toughness of the mechanical component arereduced due to the following phenomenon:

When the nitrogen-enriched layer is formed on the mechanical componentmade of steel containing at least 3.75 mass % of chromium by plasmanitriding as hereinabove described, the quantity of nitrogen on thesurface layer portion exceeds the solubility limit (solubility limitincluding nitrogen contained in precipitates) of the steel constitutingthe mechanical component. Therefore, nitrides of iron (Fe₃N, Fe₄N etc.)precipitated along grain boundaries are formed in the steel constitutingthe mechanical component. Nitrides of iron each having an aspect ratioof at least 2 and a length of at least 7.5 μm (nitrides of iron eachhaving an aspect ratio of at least 2 and a length of at least 7.5 μmformed along grain boundaries are hereinafter referred to as grainboundary precipitates) may serve as starting points of flaking andfracture.

More specifically, when stress is repetitively applied to a mechanicalcomponent having grain boundary precipitates formed therein, the stressmay be concentrated on the grain boundary precipitates, to causecracking. This cracking progresses to result in flaking or fracture,leading to reduction in fatigue resistance of the mechanical component.When impactive stress is applied to the mechanical component having thegrain boundary precipitates formed therein, the grain boundaryprecipitates promote formation or progress of cracking, and hencetoughness may be reduced. In other words, an excess quantity of nitrogenpenetrates into the surface layer portion of the mechanical component toform the grain boundary precipitates, and the fatigue resistance ortoughness of the mechanical component can be reduced due to the grainboundary precipitates.

In the mechanical component according to one aspect of the presentinvention, on the other hand, the nitrogen-enriched layer having thenitrogen concentration of at least 0.05 mass % is formed on the regionincluding the surface of the mechanical component made of the steelhaving the proper component composition and the total of the carbonconcentration and the nitrogen concentration in the nitrogen-enrichedlayer is set in the proper range, whereby formation of grain boundaryprecipitates can be suppressed. As a result, a mechanical component madeof steel containing at least 3.75 mass % of chromium and provided with anitrogen-enriched layer on a surface layer portion thereof whilesufficiently ensuring fatigue resistance and toughness can be providedaccording to the mechanical component of the present invention. Thereasons why the components of the steel constituting the presentmechanical component and the concentrations of nitrogen and carbon inthe nitrogen-enriched layer are limited in the aforementioned rangeswill now be described.

Carbon Content: at least 0.77 mass % and not more than 0.85 mass %

If the carbon content is less than 0.77 mass % in the steel constitutingthe mechanical component, there can arise such a problem that sufficientmatrix hardness cannot be attained. If the carbon content exceeds 0.85mass %, on the other hand, there can arise such a problem that a coarsecarbide (cementite: Fe₃C) is formed. Therefore, the carbon content mustbe set to at least 0.77 mass % and not more than 0.85 mass %.

Silicon Content: at least 0.01 mass % and not more than 0.25 mass %

If the silicon content is less than 0.01 mass % in the steelconstituting the mechanical component, there can arise such a problemthat the production cost for the steel is increased. If the siliconcontent exceeds 0.25 mass %, on the other hand, there can arise suchproblems that hardness of the material is increased and cold workabilityis reduced. Therefore, the silicon content must be set to at least 0.01mass % and not more than 0.25 mass %.

Manganese Content: at least 0.01 mass % and not more than 0.35 mass %

If the manganese content is less than 0.01 mass % in the steelconstituting the mechanical component, there can arise such a problemthat the production cost for the steel is increased. If the manganesecontent exceeds 0.35 mass %, on the other hand, there can arise suchproblems that the hardness of the material is increased and the coldworkability is reduced. Therefore, the manganese content must be set toat least 0.01 mass % and not more than 0.35 mass %.

Nickel Content: at least 0.01 mass % and not more than 0.15 mass %

If the nickel content is less than 0.01 mass % in the steel constitutingthe mechanical component, there can arise such a problem that theproduction cost for the steel is increased. If the nickel contentexceeds 0.15 mass %, on the other hand, there can arise such a problemthat the quantity of retained austenite is increased. Therefore, thenickel content must be set to at least 0.01 mass % and not more than0.15 mass %.

Chromium Content: at least 3.75 mass % and not more than 4.25 mass %

If the chromium content is less than 3.75 mass % in the steelconstituting the mechanical component, there can arise such a problemthat tempering softening resistance is reduced. If the chromium contentexceeds 4.25 mass %, on the other hand, there can arise such a problemthat solid solution of a carbide is inhibited. Therefore, the chromiumcontent must be set to at least 3.75 mass % and not more than 4.25 mass%.

Molybdenum Content: at least 4 mass % and not more than 4.5 mass %

If the molybdenum content is less than 4 mass % in the steelconstituting the mechanical component, there can arise such a problemthat the tempering softening resistance is reduced. If the molybdenumcontent exceeds 4.5 mass %, on the other hand, there can arise such aproblem that the production cost for the steel is increased. Therefore,the molybdenum content must be set to at least 4 mass % and not morethan 4.5 mass %.

Vanadium Content: at least 0.9 mass % and not more than 1.1 mass %

If the vanadium content is less than 0.9 mass % in the steelconstituting the mechanical component, there can arise such problemsthat the tempering softening resistance is reduced and an effect ofrefinement of the structure resulting from addition of vanadium isreduced. If the vanadium content exceeds 1.1 mass %, on the other hand,there can arise such a problem that the production cost for the steel isincreased. Therefore, the vanadium content must be set to at least 0.9mass % and not more than 1.1 mass %.

Nitrogen Concentration in Nitrogen-Enriched Layer: at least 0.05 mass %

In order to supply sufficient hardness to the surface layer portion forensuring wear resistance etc. in the mechanical component made of theaforementioned steel, the nitrogen-enriched layer having the nitrogenconcentration of at least 0.05 mass % must be formed on the regionincluding the surface. In order to further improve the wear resistanceetc., the nitrogen concentration in the surface of the mechanicalcomponent is preferably at least 0.15 mass %.

Total of Nitrogen Concentration and Carbon Concentration inNitrogen-Enriched Layer: at least 0.82 mass % and not more than 1.9 mass%

In order to supply sufficient hardness to the surface layer portion forensuring the wear resistance etc. in the mechanical component made ofthe aforementioned steel, it is important to control not only thenitrogen concentration but also the carbon concentration. The inventorshave found that it is difficult to supply sufficient hardness to thesurface layer portion for ensuring the wear resistance etc. if the totalof the nitrogen concentration and the carbon concentration in thenitrogen-enriched layer is less than 0.82 mass %. Therefore, the totalof the nitrogen concentration and the carbon concentration in thenitrogen-enriched layer must be set to at least 0.82 mass %. In order toeasily supply sufficient hardness to the surface layer portion forensuring the wear resistance etc., the total of the nitrogenconcentration and the carbon concentration in the nitrogen-enrichedlayer is preferably set to at least 0.97 mass %.

On the other hand, grain boundary precipitates are easily formed if thenitrogen concentration in the surface layer portion is increased in themechanical component made of the aforementioned steel, and this tendencyis further strengthened if the carbon concentration is increased. Theinventors have found that it is difficult to suppress formation of grainboundary precipitates if the total of the nitrogen concentration and thecarbon concentration in the nitrogen-enriched layer exceeds 1.9 mass %.Therefore, the total of the nitrogen concentration and the carbonconcentration in the nitrogen-enriched layer must be set to not morethan 1.9 mass %. In order to further suppress formation of grainboundary precipitates, the total of the nitrogen concentration and thecarbon concentration in the nitrogen-enriched layer is preferably set tonot more than 1.7 mass %. The carbon concentration and the nitrogenconcentration denote concentrations in the matrix (mother phase) whichis a region other than carbides of iron, chromium etc.

Preferably in the above mechanical component, the thickness of theaforementioned nitrogen-enriched layer is at least 0.11 mm. In bearings,hubs, constant velocity joints, gears and other mechanical components,the strength of the surface and a portion immediately under the surface,more specifically a region within 0.11 mm in distance from the surfacemay generally be important. Therefore, the thickness of theaforementioned nitrogen-enriched layer is so set to at least 0.11 mmthat sufficient strength can be supplied to the mechanical component. Inorder to render the strength of the mechanical component moresufficient, the thickness of the aforementioned nitrogen-enriched layeris preferably at least 0.15 mm.

Preferably in the above mechanical component, the aforementionednitrogen-enriched layer has hardness of at least 830 HV. The hardness ofthe nitrogen-enriched layer formed on the surface layer portion is soset to at least 830 HV that the strength of the mechanical component canbe more reliably ensured.

Preferably in the above mechanical component, the number of nitrides ofiron each having an aspect ratio of at least 2 and a length of at least7.5 μm is not more than one in five fields of view of square regions of150 μm on each side when the aforementioned nitrogen-enriched layer isobserved with a microscope.

As hereinabove described, grain boundary precipitates of nitride of ironeach having an aspect ratio of at least 2 and a length of at least 7.5μm may reduce characteristics such as the fatigue resistance and thetoughness of the mechanical components. The inventors have investigatedthe relation between the fatigue resistance and the number density ofgrain boundary precipitates as to a mechanical component made of steelhaving the aforementioned component composition, to find that thefatigue resistance of the mechanical component is reduced if the grainboundary precipitates are present in a number density exceeding one infive fields of view of square regions of 150 μm on each side when theaforementioned nitrogen-enriched layer is observed with a microscope. Ifthe number of grain boundary precipitates is not more than one in fivefields of view of square regions of 150 μm on each side when thenitrogen-enriched layer is observed with a microscope, therefore, thefatigue resistance of the mechanical component can be improved. In orderto further improve the fatigue resistance of the mechanical component,the number of the aforementioned grain boundary precipitates ispreferably not more than one in 60 fields of view of square regions of150 μm on each side.

The present mechanical component may be used as a component configuringa bearing. The present mechanical component that has a surface layerportion nitrided and thus reinforced and also reduces/prevents grainboundary precipitates is suitable as a component configuring a bearing,which is a mechanical component required to have fatigue resistance,wear resistance, and the like.

The above mechanical component may be used to configure a rollingbearing including a bearing ring and a rolling element arranged incontact with the bearing ring on an annular raceway. More specifically,one of or preferably both the bearing ring and the rolling elementis/are the above mechanical component(s). The present mechanicalcomponent that has a surface layer portion nitrided and thus reinforcedand also reduces/prevents grain boundary precipitates allows the rollingbearing to have long life.

The concentrations of nitrogen and carbon in the nitrogen-enriched layercan be investigated by EPMA (electron probe microanalysis), for example.The number density of the aforementioned nitrides of iron (grainboundary precipitates) can be investigated as follows, for example:First, the mechanical component is cut along a section perpendicular tothe surface thereof, and this section is polished. Then, the section isetched with a proper etchant, and the nitrogen-enriched layer isthereafter observed with an SEM (scanning electron microscope) or anoptical microscope and photographed. A square field of view having eachside of 150 μm and one side corresponding to the surface is analyzedwith an image analyzer, for investigating the number of grain boundaryprecipitates. This analysis is randomly performed in at least fivefields of view, for calculating the number of grain boundaryprecipitates in five fields of view.

The rolling bearing in one aspect of the present invention includes arace member and a plurality of rolling elements disposed in contact withthe race member on an annular raceway. The race member is the mechanicalcomponent in one aspect above, and the rolling element is formed ofceramic.

In the rolling bearing according to one aspect of the present invention,the rolling elements are made of ceramics. Thus, smearing is suppressed,and the race member and the rolling elements coming into contact witheach other are made of different materials, whereby the seizureresistance is improved. Further, ceramics harder than steel is soemployed as the material for the rolling elements that the durability ofthe rolling elements in a contaminated environment is improved. As aresult, smearing resistance, and durability in an environment havingforeign matters introduced therein are enhanced, and simultaneously,durability in an environment with insufficient lubricity, such asdry-run performance, is enhanced. In addition, the rolling elements areso made of ceramics that reduction in hardness of the rolling elementsin a high-temperature environment is suppressed. As the ceramicsconstituting the rolling elements, silicon nitride, sialon, alumina orzirconia, for example, can be employed.

In the rolling bearing according to one aspect of the present invention,as hereinabove described, the race member is made of the steelcontaining at least 3.75 mass % of chromium, and the rolling element isformed of ceramic, whereby reduction in hardness of the bearingcomponents in a high-temperature environment is suppressed. Further, thenitrogen-enriched layer having the total of the carbon concentration andthe nitrogen concentration set in the proper range is formed on thesurface layer portion of the race member made of the steel having theproper component composition, and the rolling element is formed ofceramic, whereby the durability of the bearing components in acontaminated environment is improved. Furthermore the race member formedof steel in combination with the ceramic rolling element can improvesmearing resistance and dry-run performance. Consequently, a rollingbearing capable of attaining not only suppression of reduction inhardness of the bearing components in a high-temperature environment,improvement in durability in a contaminated environment and improvementin smearing resistance but also improvement of the dry-run performancecan be provided according to the present invention.

The rolling bearing according to one aspect of the present invention maybe employed as a bearing supporting a rotating member which is a mainshaft or a member rotating upon rotation of the main shaft to berotatable with respect to a member adjacent to the rotating member in agas turbine engine. To such a bearing supporting the rotating member(the main shaft or the member rotating upon rotation of the main shaft)in the gas turbine engine, suppression of reduction in hardness ofbearing components in a high temperature environment, improvement ofdurability in a contaminated environment, improvement in smearingresistance and improvement of dry-run performance are required.Therefore, the rolling bearing according to the present inventioncapable of attaining not only suppression of reduction in hardness ofbearing components in a high-temperature environment, improvement indurability in a contaminated environment and improvement in smearingresistance but also improvement of dry-run performance is suitable as abearing supporting a rotating member in a gas turbine engine.

A mechanical component according to another aspect of the presentinvention is constituted of steel containing at least 0.11 mass % andnot more than 0.15 mass % of carbon, at least 0.1 mass % and not morethan 0.25 mass % of silicon, at least 0.15 mass % and not more than 0.35mass % of manganese, at least 3.2 mass % and not more than 3.6 mass % ofnickel, at least 4 mass % and not more than 4.25 mass % of chromium, atleast 4 mass % and not more than 4.5 mass % of molybdenum and at least1.13 mass % and not more than 1.33 mass % of vanadium with a remainderconsisting of iron and impurity. The mechanical component is providedwith a nitrogen-enriched layer having a nitrogen concentration of atleast 0.05 mass % formed on a region including a surface, and the totalof a carbon concentration and the nitrogen concentration in thenitrogen-enriched layer is at least 0.55 mass % and not more than 1.9mass %.

In the mechanical component according to another aspect of the presentinvention, on the other hand, the nitrogen-enriched layer having thenitrogen concentration of at least 0.05 mass % is formed on the regionincluding the surface of the mechanical component made of the steelhaving the proper component composition and the total of the carbonconcentration and the nitrogen concentration in the nitrogen-enrichedlayer is set in the proper range, whereby formation of grain boundaryprecipitates can be suppressed. As a result, a mechanical component madeof steel containing at least 4 mass % of chromium and provided with anitrogen-enriched layer on a surface layer portion thereof whilesufficiently ensuring fatigue resistance and toughness can be providedaccording to the mechanical component of the present invention. Thereasons why the components of the steel constituting the presentmechanical component and the concentrations of nitrogen and carbon inthe nitrogen-enriched layer are limited in the aforementioned rangeswill now be described.

Carbon Content: at least 0.11 mass % and not more than 0.15 mass %

If the carbon content is less than 0.11 mass % in the steel constitutingthe mechanical component, there can arise such a problem that theproduction cost for the steel is increased. If the carbon contentexceeds 0.15 mass %, on the other hand, there can arise such problemsthat core hardness is increased and toughness is reduced. Therefore, thecarbon content must be set to at least 0.11 mass % and not more than0.15 mass %.

Silicon Content: at least 0.1 mass % and not more than 0.25 mass %

If the silicon content is less than 0.1 mass % in the steel constitutingthe mechanical component, there can arise such a problem that theproduction cost for the steel is increased. If the silicon contentexceeds 0.25 mass %, on the other hand, there can arise such problemsthat hardness of the material is increased and cold workability isreduced. Therefore, the silicon content must be set to at least 0.1 mass% and not more than 0.25 mass %.

Manganese Content: at least 0.15 mass % and not more than 0.35 mass %

If the manganese content is less than 0.15 mass % in the steelconstituting the mechanical component, there can arise such a problemthat the production cost for the steel is increased. If the manganesecontent exceeds 0.35 mass %, on the other hand, there can arise suchproblems that the hardness of the material is increased and the coldworkability is reduced. Therefore, the manganese content must be set toat least 0.15 mass % and not more than 0.35 mass %.

Nickel Content: at least 3.2 mass % and not more than 3.6 mass %

If the nickel content is less than 3.2 mass % in the steel constitutingthe mechanical component, there can arise such a problem that effects ofimproving corrosion resistance, hardness and toughness are reduced. Ifthe nickel content exceeds 3.6 mass %, on the other hand, there canarise such a problem that the quantity of retained austenite isincreased. Therefore, the nickel content must be set to at least 3.2mass % and not more than 3.6 mass %.

Chromium Content: at least 4 mass % and not more than 4.25 mass %

If the chromium content is less than 4 mass % in the steel constitutingthe mechanical component, there can arise such a problem that temperingsoftening resistance is reduced. If the chromium content exceeds 4.25mass %, on the other hand, there can arise such a problem that solidsolution of a carbide is inhibited. Therefore, the chromium content mustbe set to at least 4 mass % and not more than 4.25 mass %.

Molybdenum Content: at least 4 mass % and not more than 4.5 mass %

If the molybdenum content is less than 4 mass % in the steelconstituting the mechanical component, there can arise such a problemthat the tempering softening resistance is reduced. If the molybdenumcontent exceeds 4.5 mass %, on the other hand, there can arise such aproblem that the production cost for the steel is increased. Therefore,the molybdenum content must be set to at least 4 mass % and not morethan 4.5 mass %.

Vanadium Content: at least 1.13 mass % and not more than 1.33 mass %

If the vanadium content is less than 1.13 mass % in the steelconstituting the mechanical component, there can arise such problemsthat the tempering softening resistance is reduced and an effect ofrefinement of a microstructure resulting from addition of vanadium isreduced. If the vanadium content exceeds 1.33 mass %, on the other hand,there can arise such a problem that the production cost for the steel isincreased. Therefore, the vanadium content must be set to at least 1.13mass % and not more than 1.33 mass %.

Nitrogen Concentration in Nitrogen-Enriched Layer: at least 0.05 mass %

In order to supply sufficient hardness to the surface layer portion forensuring wear resistance etc. in the mechanical component made of theaforementioned steel, the nitrogen-enriched layer having the nitrogenconcentration of at least 0.05 mass % must be formed on the regionincluding the surface. In order to further improve the wear resistanceetc., the nitrogen concentration in the surface of the mechanicalcomponent is preferably at least 0.15 mass %.

Total of Nitrogen Concentration and Carbon Concentration inNitrogen-Enriched Layer: at least 0.55 mass % and not more than 1.9 mass%

In order to supply sufficient hardness to the surface layer portion forensuring the wear resistance etc. in the mechanical component made ofthe aforementioned steel, it is important to control not only thenitrogen concentration but also the carbon concentration. The inventorshave found that it is difficult to supply sufficient hardness to thesurface layer portion for ensuring the wear resistance etc. if the totalof the nitrogen concentration and the carbon concentration in thenitrogen-enriched layer is less than 0.55 mass %. Therefore, the totalof the nitrogen concentration and the carbon concentration in thenitrogen-enriched layer must be set to at least 0.55 mass %. In order toeasily supply sufficient hardness to the surface layer portion forensuring the wear resistance etc., the total of the nitrogenconcentration and the carbon concentration in the nitrogen-enrichedlayer is preferably set to at least 0.7 mass %.

On the other hand, grain boundary precipitates are easily formed if thenitrogen concentration in the surface layer portion is increased in themechanical component made of the aforementioned steel, and this tendencyis further strengthened if the carbon concentration is increased. Theinventors have found that it is difficult to suppress formation of grainboundary precipitates if the total of the nitrogen concentration and thecarbon concentration in the nitrogen-enriched layer exceeds 1.9 mass %.Therefore, the total of the nitrogen concentration and the carbonconcentration in the nitrogen-enriched layer must be set to not morethan 1.9 mass %. In order to further suppress formation of grainboundary precipitates, the total of the nitrogen concentration and thecarbon concentration in the nitrogen-enriched layer is preferably set tonot more than 1.7 mass %. The carbon concentration and the nitrogenconcentration denote concentrations in the matrix (mother phase) whichis a region other than carbides and nitrides of iron, chromium etc.

Preferably in the mechanical component in the above other aspect, thethickness of the aforementioned nitrogen-enriched layer is at least 0.11mm. In bearings, hubs, constant velocity joints, gears and othermechanical components, the strength of the surface and a portionimmediately under the surface, more specifically a region within 0.11 mmin distance from the surface may generally be important. Therefore, thethickness of the aforementioned nitrogen-enriched layer is so set to atleast 0.11 mm that sufficient strength can be supplied to the mechanicalcomponent. In order to render the strength of the mechanical componentmore sufficient, the thickness of the aforementioned nitrogen-enrichedlayer is preferably at least 0.15 mm.

Preferably in the mechanical component in the above other aspect, theaforementioned nitrogen-enriched layer has hardness of at least 800 HV.The hardness of the nitrogen-enriched layer formed on the surface layerportion is so set to at least 800 HV that the strength of the mechanicalcomponent can be more reliably ensured.

Preferably in the mechanical component in the above other aspect, thenumber of nitrides of iron each having an aspect ratio of at least 2 anda length of at least 7.5 μm is not more than one in five fields of viewof square regions of 150 μm on each side when the aforementionednitrogen-enriched layer is observed with a microscope.

As hereinabove described, grain boundary precipitates of nitride of ironeach having an aspect ratio of at least 2 and a length of at least 7.5μm may reduce characteristics such as the fatigue resistance and thetoughness of the mechanical components. The inventors have investigatedthe relation between the fatigue resistance and the number density ofgrain boundary precipitates as to a mechanical component made of steelhaving the aforementioned component composition, to find that thefatigue resistance of the mechanical component is reduced if the grainboundary precipitates are present in a number density exceeding one infive fields of view of square regions of 150 μm on each side when theaforementioned nitrogen-enriched layer is observed with a microscope. Ifthe number of grain boundary precipitates is not more than one in fivefields of view of square regions of 150 μm on each side when thenitrogen-enriched layer is observed with a microscope, therefore, thefatigue resistance of the mechanical component can be improved. In orderto further improve the fatigue resistance of the mechanical component,the number of the aforementioned grain boundary precipitates ispreferably not more than one in 60 fields of view of square regions of150 μm on each side.

The mechanical component of the present invention in another aspect maybe used as a component configuring a bearing. The present mechanicalcomponent that has a surface layer portion nitrided and thus reinforcedand also reduces/prevents grain boundary precipitates is suitable as acomponent configuring a bearing, which is a mechanical componentrequired to have fatigue resistance, wear resistance, and the like.

The mechanical component in the above other aspect may be used toconfigure a rolling bearing including a bearing ring and a rollingelement arranged in contact with the bearing ring on an annular raceway.More specifically, one of or preferably both the bearing ring and therolling element is/are the above mechanical component(s). The presentmechanical component that has a surface layer portion nitrided and thusreinforced and also reduces/prevents grain boundary precipitates allowsthe rolling bearing to have long life.

The rolling bearing in another aspect of the present invention includesa race member and a plurality of rolling elements disposed in contactwith the race member on an annular raceway. The race member is themechanical component in the above other aspect, and the rolling elementis formed of ceramic.

In the rolling bearing according to another aspect of the presentinvention, the rolling elements are made of ceramics. Thus, smearing issuppressed, and the race member and the rolling elements coming intocontact with each other are made of different materials, whereby theseizure resistance is improved. Further, ceramics harder than steel isso employed as the material for the rolling elements that the durabilityof the rolling elements in a contaminated environment is improved. As aresult, smearing resistance, and durability in an environment havingforeign matters introduced therein are enhanced, and simultaneously,durability in an environment with insufficient lubricity, such asdry-run performance, is enhanced. In addition, the rolling elements areso made of ceramics that reduction in hardness of the rolling elementsin a high-temperature environment is suppressed. As the ceramicsconstituting the rolling elements, silicon nitride, sialon, alumina orzirconia, for example, can be employed.

In the rolling bearing according to another aspect of the presentinvention, as hereinabove described, the race member is made of thesteel containing at least 4 mass % of chromium, and the rolling elementis formed of ceramic, whereby reduction in hardness of the bearingcomponents in a high-temperature environment is suppressed. Further, thenitrogen-enriched layer having the total of the carbon concentration andthe nitrogen concentration set in the proper range is formed on thesurface layer portion of the race member made of the steel having theproper component composition, and the rolling element is formed ofceramic, whereby the durability of the bearing components in acontaminated environment is improved. Furthermore the race member formedof steel in combination with the ceramic rolling element can improvesmearing resistance and dry-run performance. Consequently, a rollingbearing capable of attaining not only suppression of reduction inhardness of the bearing components in a high-temperature environment,improvement in durability in a contaminated environment and improvementin smearing resistance but also improvement of the dry-run performancecan be provided according to the present invention.

The rolling bearing according to another aspect of the present inventionas described above may be employed as a bearing supporting a rotatingmember which is a main shaft or a member rotating upon rotation of themain shaft to be rotatable with respect to a member adjacent to therotating member in a gas turbine engine. To such a bearing supportingthe rotating member (the main shaft or the member rotating upon rotationof the main shaft) in the gas turbine engine, suppression of reductionin hardness of bearing components in a high temperature environment,improvement of durability in a contaminated environment, improvement insmearing resistance and improvement of dry-run performance are required.Therefore, the rolling bearing according to the present inventioncapable of attaining not only suppression of reduction in hardness ofbearing components in a high-temperature environment, improvement indurability in a contaminated environment and improvement in smearingresistance but also improvement of dry-run performance is suitable as abearing supporting a rotating member in a gas turbine engine.

EFFECTS OF THE INVENTION

As is apparent from the above description, the present invention canthus provide a mechanical component that is formed of steel containingat least 3.75 mass % of chromium, and also has a surface layer portionhaving a nitrogen enriched layer, and also ensures sufficient fatigueresistance and toughness. Furthermore, the present invention can providea rolling bearing capable of attaining not only suppression of reductionin hardness of bearing components in a high-temperature environment andimprovement in durability in a contaminated environment and improvementin smearing resistance but also improvement of dry-run performance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross section of a configuration of a deep grooveball bearing including a mechanical component in a first embodiment ofthe present invention.

FIG. 2 is an enlarged schematic partial cross section of a principalpart of FIG. 1.

FIG. 3 is a schematic cross section of a configuration of a thrustneedle roller bearing including a mechanical component in a firstexemplary variation.

FIG. 4 is an enlarged schematic partial cross section of a principalpart of FIG. 3.

FIG. 5 is a schematic cross section of a configuration of a constantvelocity joint including a mechanical component in a second exemplaryvariation.

FIG. 6 is a schematic cross section taken along a line VI-VI shown inFIG. 5.

FIG. 7 is a schematic cross section of the FIG. 5 constant velocityjoint forming an angle.

FIG. 8 is an enlarged schematic partial cross section of a principalpart of FIG. 5.

FIG. 9 is an enlarged schematic partial cross section of a principalpart of FIG. 6.

FIG. 10 generally illustrates a method of producing the mechanicalcomponent and a method of producing a mechanical element including themechanical component in the first embodiment.

FIG. 11 is a diagram for illustrating the details of a heat treatmentstep included in the method of producing the mechanical component.

FIG. 12 is a schematic diagram showing the structure of a turbofanengine to which a rolling bearing of the present invention isapplicable.

FIG. 13 is a schematic cross section of the structure of a three-pointcontact ball bearing.

FIG. 14 is an enlarged schematic partial cross section of a principalpart of FIG. 13.

FIG. 15 is a schematic cross section of the structure of a cylindricalroller bearing.

FIG. 16 is an enlarged schematic partial cross section of a principalpart of FIG. 15.

FIG. 17 is a flow chart outlining a method of producing a rollingbearing.

FIG. 18 generally illustrates a method of producing the mechanicalcomponent and a method of producing a mechanical element including themechanical component in a third embodiment of the present invention.

FIG. 19 is a diagram for illustrating the details of a heat treatmentstep included in the method of producing the mechanical component.

FIG. 20 is a flow chart outlining a method of producing a rollingbearing.

FIG. 21 is an optical micrograph of a microstructure around the surfaceof Example A of the present invention.

FIG. 22 illustrates a hardness distribution around the surface ofExample A of the present invention.

FIG. 23 illustrates distributions of concentrations of carbon andnitrogen around the surface of Example A of the present invention.

FIG. 24 is an optical micrograph of a microstructure around the surfaceof comparative example A.

FIG. 25 illustrates a hardness distribution around the surface ofcomparative example A.

FIG. 26 illustrates distributions of concentrations of carbon andnitrogen around the surface of comparative example A.

FIG. 27 is a diagram (Avrami's plot) showing the relation between heattreatment times at respective heating temperatures in a diffusion stepand the hardness levels of a mother phase.

FIG. 28 illustrates hardness distributions on surface layer portions ofspecimens.

FIG. 29 is an optical micrograph of a microstructure around the surfaceof Example B of the present invention.

FIG. 30 illustrates a hardness distribution around the surface ofExample B of the present invention.

FIG. 31 illustrates distributions of concentrations of carbon andnitrogen around the surface of Example B of the present invention.

FIG. 32 is an optical micrograph of a microstructure around the surfaceof comparative example B.

FIG. 33 illustrates a hardness distribution around the surface ofcomparative example B.

FIG. 34 illustrates distributions of concentrations of carbon andnitrogen around the surface of comparative example B.

FIG. 35 is a diagram (Avrami's plot) showing the relation between heattreatment times at respective heating temperatures in a diffusion stepand the hardness levels of carburized layers.

FIG. 36 illustrates hardness distributions on surface layer portions ofspecimens.

DESCRIPTION OF THE REFERENCE SIGNS

1: deep groove ball bearing, 2: thrust needle roller bearing, 3:constant velocity joint, 4: three point contact ball bearing, 5:cylindrical roller bearing, 11: outer ring, 11A: outer ring racewaysurface, 11B: outer ring nitrogen-enriched layer, 12: inner ring, 12A:inner ring raceway surface, 12B: inner ring nitrogen-enriched layer, 13:ball, 13A: ball rolling contact surface, 13B: ball nitrogen-enrichedlayer, 14, 24: cage, 21: bearing washer, 21A: bearing washer racewaysurface, 21B: bearing washer nitrogen-enriched layer, 23A: rollerrolling contact surface, 23B: roller nitrogen-enriched layer, 31: innerrace, 31A: inner race ball groove, 31B: inner race nitrogen-enrichedlayer, 32: outer race, 32A: outer race ball groove, 32B: outer racenitrogen-enriched layer, 33: ball, 33A: ball rolling contact surface,33B: ball nitrogen-enriched layer, 34: cage, 35, 36: shaft, 41, 51:outer ring, 41A, 51A: outer ring raceway surface, 41B, 51B: outer ringnitrogen-enriched layer, 42, 52: inner ring, 42A, 52A: inner ringraceway surface, 42B, 52B: inner ring nitrogen-enriched layer, 421:first inner ring, 421A: first inner ring raceway surface, 422: secondinner ring, 422A: second inner ring raceway surface, 43: ball, 43A: ballrolling contact surface, 44, 54: cage, 53: roller, 53A: roller rollingcontact surface, 70: turbo fan engine, 71: compression portion, 72:combustion portion, 73: turbine portion, 74: low-pressure main shaft,75: fan, 75A: fan blade, 76: fan nacelle, 77: high-pressure main shaft,78: core cowl, 79: bypass passage, 81: compressor, 81A: low-pressurecompressor, 81B: high-pressure compressor, 82: combustion chamber, 83:turbine, 83A: low-pressure turbine, 83B: high-pressure turbine, 84:turbine nozzle, 87: turbine blade, 88: compressor blade, 89: rollingbearing, 90: grain boundary precipitates.

BEST MODES FOR CARRYING OUT THE INVENTION

Hereinafter reference will be made to the drawings to describe thepresent invention in embodiments. In the figures, identical orcorresponding components are identically denoted and will not bedescribed repeatedly.

First Embodiment

Initially the present invention in a first embodiment will be described.

With reference to FIG. 1, a deep groove ball bearing 1 includes anannular outer ring 11, an annular inner ring 12 arranged to be innerthan outer ring 11, and rolling elements implemented as a plurality ofballs 13 arranged between outer and inner rings 11 and 12 and held in anannular cage 14. Outer ring 11 has an inner circumferential surfacehaving an outer ring raceway surface 11A and inner ring 12 has an outercircumferential surface having an inner ring raceway surface 12A. Outerring 11 and inner ring 12 are disposed such that inner ring racewaysurface 12A and outer ring raceway surface 11A face each other. Theplurality of balls 13 are held in a rollable manner on an annularraceway, with their rolling contact surfaces 13A in contact with innerring raceway surface 12A and outer ring raceway surface 11A, disposed ata predetermined pitch in the circumferential direction by means of cage14. By such a configuration, outer ring 11 and inner ring 12 of deepgroove ball bearing 1 can be rotated relative to each other.

Outer ring 11, inner ring 12 and ball 13 serving as mechanicalcomponents are made of steel containing at least 0.77 mass % and notmore than 0.85 mass % of carbon, at least 0.01 mass % and not more than0.25 mass % of silicon, at least 0.01 mass % and not more than 0.35 mass% of manganese, at least 0.01 mass % and not more than 0.15 mass % ofnickel, at least 3.75 mass % and not more than 4.25 mass % of chromium,at least 4 mass % and not more than 4.5 mass % of molybdenum and atleast 0.9 mass % and not more than 1.1 mass % of vanadium with aremainder consisting of iron and impurity. Referring to FIG. 2, an outerring nitrogen-enriched layer 11B, an inner ring nitrogen-enriched layer12B and a ball nitrogen-enriched layer 13B having nitrogenconcentrations of at least 0.05 mass % are formed on regions includingouter ring raceway surface 11A, inner ring raceway surface 12A and ballrolling contact surface 13A which are the surfaces of outer ring 11,inner ring 12 and ball 13. Further, the totals of carbon concentrationsand the nitrogen concentrations in outer ring nitrogen-enriched layer11B, inner ring nitrogen-enriched layer 12B and ball nitrogen-enrichedlayer 13B are at least 0.82 mass % and not more than 1.9 mass %. Theaforementioned impurity includes unavoidable impurity such as thatderived from the raw materials for the steel, that mixed in productionsteps and the like.

Outer ring 11, inner ring 12 and ball 13 serving as the mechanicalcomponents according to the present embodiment are made of the steelhaving the aforementioned proper component composition, and outer ringnitrogen-enriched layer 11B, inner ring nitrogen-enriched layer 12B andball nitrogen-enriched layer 13B having the nitrogen concentrations ofat least 0.05 mass % are formed on the regions including outer ringraceway surface 11A, inner ring raceway surface 12A and ball rollingcontact surface 13A formed on the surfaces thereof. The totals of thecarbon concentrations and the nitrogen concentrations in outer ringnitrogen-enriched layer 11B, inner ring nitrogen-enriched layer 12B andball nitrogen-enriched layer 13B are so set in the proper range of atleast 0.82 mass % and not more than 1.9 mass % that sufficient hardnessis supplied to surface layer portions thereof and formation of grainboundary precipitates is suppressed. Consequently, outer ring 11, innerring 12 and ball 13 serving as the mechanical components in the presentembodiment are made of steel containing at least 3.75 mass % of chromiumand provided with nitrogen-enriched layers on the surface layer portionsthereof, while sufficiently ensuring fatigue resistance and toughness.Furthermore, outer ring 11, inner ring 12 and ball 13 allow a rollingbearing implemented as deep groove ball bearing 1 to have a long life.

Preferably, the thicknesses of outer ring nitrogen-enriched layer 11B,inner ring nitrogen-enriched layer 12B and ball nitrogen-enriched layer13B formed on outer ring 11, inner ring 12 and ball 13 are at least 0.11mm. Thus, sufficient strength is supplied to outer ring 11, inner ring12 and ball 13.

Preferably, outer ring nitrogen-enriched layer 11B, inner ringnitrogen-enriched layer 12B and ball nitrogen-enriched layer 13B havehardness of at least 830 HV. Thus, the strength of outer ring 11, innerring 12 and ball 13 can be more reliably ensured.

Preferably, the number of nitrides of iron each having an aspect ratioof at least 2 and a length of at least 7.5 μm is not more than one infive fields of view of square regions of 150 μm on each side when outerring nitrogen-enriched layer 11B, inner ring nitrogen-enriched layer 12Band ball nitrogen-enriched layer 13B are observed with a microscope.Thus, the fatigue resistance of outer ring 11, inner ring 12 and ball 13can be improved.

Hereinafter reference will be made to FIG. 3 and FIG. 4 to describe athrust needle roller bearing in a first exemplary variation of thepresent embodiment.

With reference to FIG. 3, thrust needle roller bearing 2 includes a pairof bearing washers 21 in the form of a disk, serving as a rollingcontact member arranged such that their respective, one main surfacesface each other, a plurality of needle rollers 23 serving as a rollingcontact member, and an annular cage 24. The plurality of needle rollers23 are held in a rollable manner on an annular raceway, with their outercircumferential surfaces i.e., a roller rolling contact surface 23A, incontact with bearing washer raceway surface 21A formed at the mainsurfaces of the pair of bearing washers 21 facing each other, disposedat a predetermined pitch in the circumferential direction by means ofcage 24. By such a configuration, the pair of bearing washers 21 ofthrust needle roller bearing 2 can be rotated relative to each other.

Herein, with reference to FIG. 4, in the present exemplary variation,bearing washer 21 and needle roller 23 of thrust needle roller bearing 2correspond to outer and inner rings 11 and 12 and ball 13 of deep grooveball bearing 1, respectively, and the former is similar to the latter inconfiguration and effect. More specifically, bearing washer 21 andneedle roller 23 serving as the mechanical components are made of steelcontaining at least 0.77 mass % and not more than 0.85 mass % of carbon,at least 0.01 mass % and not more than 0.25 mass % of silicon, at least0.01 mass % and not more than 0.35 mass % of manganese, at least 0.01mass % and not more than 0.15 mass % of nickel, at least 3.75 mass % andnot more than 4.25 mass % of chromium, at least 4 mass % and not morethan 4.5 mass % of molybdenum and at least 0.9 mass % and not more than1.1 mass % of vanadium with a remainder consisting of iron and impurity.Referring to FIG. 4, a bearing washer nitrogen-enriched layer 21B and aroller nitrogen-enriched layer 23B having nitrogen concentrations of atleast 0.05 mass % are formed on regions including bearing washer racewaysurface 21A and roller rolling contact surface 23A which are thesurfaces of bearing washer 21 and needle roller 23. Further, the totalsof carbon concentrations and the nitrogen concentrations in bearingwasher nitrogen-enriched layer 21B and roller nitrogen-enriched layer23B are at least 0.82 mass % and not more than 1.9 mass %.

Bearing washer 21 and needle roller 23 serving as the mechanicalcomponents in the present exemplary variation are made of the steelhaving the aforementioned proper component composition, and bearingwasher nitrogen-enriched layer 21B and roller nitrogen-enriched layer23B having the nitrogen concentrations of at least 0.05 mass % areformed on the regions including bearing washer raceway surface 21A androller rolling contact surface 23A formed on the surfaces thereof. Thetotals of the carbon concentrations and the nitrogen concentrations inbearing washer nitrogen-enriched layer 21B and roller nitrogen-enrichedlayer 23B are so set in the proper range of at least 0.82 mass % and notmore than 1.9 mass % that sufficient hardness is supplied to surfacelayer portions thereof and formation of grain boundary precipitates issuppressed. Consequently, bearing washer 21 and needle roller 23 servingas the mechanical components in the present exemplary variation are madeof steel containing at least 3.75 mass % of chromium and provided withnitrogen-enriched layers on the surface layer portions thereof, whilesufficiently ensuring fatigue resistance and toughness.

Furthermore, bearing washer 21 and needle roller 23 allow a rollingbearing implemented as thrust needle roller bearing 2 to have a longlife.

With reference to FIG. 5 to FIG. 9, the present embodiment in a secondexemplary variation provides a constant velocity joint, as will bedescribed hereinafter. With reference to FIG. 5 and FIG. 6, a constantvelocity joint 3 includes an inner race 31 coupled to a shaft 35, anouter race 32 arranged to surround the outer circumferential side ofinner race 31 and coupled to a shaft 36, a torque transmitting ball 33arranged between inner race 31 and outer race 32, and a cage 34 holdingball 33. Ball 33 is arranged with ball rolling contact surface 33A incontact with an inner race ball groove 31A formed at the outercircumferential surface of inner race 31 and an outer race ball groove32A formed at the inner circumferential surface of outer race 32, and isheld by cage 34 to avoid falling off.

As shown in FIG. 3, inner race ball groove 31A and outer race ballgroove 32A located at the outer circumferential surface of inner race 31and the inner circumferential surface of outer race 32, respectively,are formed in a curve (arc) with points A and B equally spaced apart atthe left and right on the axis passing through the center of shafts 35and 36 in a straight line from the joint center O on the axis as thecenter of curvature. In other words, inner race ball groove 31A andouter race ball groove 32A are formed such that the trajectory of centerP of ball 33 that rolls in contact with inner race ball groove 31A andouter race ball groove 32A corresponds to a curve (arc) with point A(inner race center A) and point B (outer race center B) as the center ofcurvature. Accordingly, ball 33 is constantly located on the bisector ofan angle (∠AOB) with respect to the axis passing through the center ofshafts 35 and 36 even when the constant velocity joint forms an angle(when the constant velocity joint operates such that the axes passingthrough the center of shafts 35 and 36 cross).

Constant velocity joint 3 operates, as will be described hereinafter.With reference to FIG. 5 and FIG. 6, when the rotation about the axis istransmitted to one of shafts 35 and 36 at constant velocity joint 3,this rotation is transmitted to the other of shafts 35 and 36 via ball33 fitted in inner race ball groove 31A and outer race ball groove 32A.In the case where shafts 35 and 36 form an angle θ as shown in FIG. 7,ball 33 is guided by inner race ball groove 31A and outer race ballgroove 32A with inner race center A and outer race center B as thecenter of curvature to be held at a position where its center P islocated on the bisector of ∠AOB. Since inner race ball groove 31A andouter race ball groove 32A are formed such that the distance from jointcenter O to inner race center A is equal to the distance from jointcenter O to outer race center B, the distance from center P of ball 33to respective inner race center A and outer race center B is equal.Thus, triangle OAP is congruent to triangle OBP. As a result, thedistances L from center P of ball 33 to shafts 35 and 36 are equal toeach other, and when one of shafts 35 and 36 rotates about the axis, theother also rotates at constant velocity. Thus, constant velocity joint 3can ensure constant velocity even in the state where shafts 35 and 36constitute an angle. Cage 34 serves, together with inner race ballgroove 31A and outer race ball groove 32A, to prevent ball 33 fromjumping out of inner race ball groove 31A and outer race ball groove 32Awhen shafts 35 and 36 rotate, and also serves to determine joint centerO of constant velocity joint 3.

Herein, in the present exemplary variation, inner and outer races 31 and32 and ball 33 of constant velocity joint 3 correspond to outer andinner rings 11 and 12 and ball 13 of deep groove ball bearing 1,respectively, and the former is similar to the latter in configurationand effect. More specifically, inner and outer races 31 and 32 and ball33 serving as the mechanical components are made of steel containing atleast 0.77 mass % and not more than 0.85 mass % of carbon, at least 0.01mass % and not more than 0.25 mass % of silicon, at least 0.01 mass %and not more than 0.35 mass % of manganese, at least 0.01 mass % and notmore than 0.15 mass % of nickel, at least 3.75 mass % and not more than4.25 mass % of chromium, at least 4 mass % and not more than 4.5 mass %of molybdenum and at least 0.9 mass % and not more than 1.1 mass % ofvanadium with a remainder consisting of iron and impurity. Referring toFIG. 8 and FIG. 9, an inner race nitrogen-enriched layer 31B, an outerrace nitrogen-enriched layer 32B, and a ball nitrogen-enriched layer 33Bhaving nitrogen concentrations of at least 0.05 mass % are formed onregions including a surface of inner race ball groove 31A, a surface ofouter race ball groove 32A, and ball rolling contact surface 33A whichare the surfaces of inner and outer races 31 and 32 and ball 33.Further, the totals of carbon concentrations and the nitrogenconcentrations in inner race nitrogen-enriched layer 31B, outer racenitrogen-enriched layer 32B, and ball nitrogen-enriched layer 33B are atleast 0.82 mass % and not more than 1.9 mass %.

Inner and outer races 31 and 32 and ball 33 serving as the mechanicalcomponents in the present exemplary variation are made of the steelhaving the aforementioned proper component composition, and inner racenitrogen-enriched layer 31B, outer race nitrogen-enriched layer 32B, andball nitrogen-enriched layer 33B having the nitrogen concentrations ofat least 0.05 mass % are formed on the regions including a surface ofinner race ball groove 31A, a surface of outer race ball groove 32A, andball rolling contact surface 33A formed on the surfaces thereof. Thetotals of the carbon concentrations and the nitrogen concentrations ininner race nitrogen-enriched layer 31B, outer race nitrogen-enrichedlayer 32B, and ball nitrogen-enriched layer 33B are so set in the properrange of at least 0.82 mass % and not more than 1.9 mass % thatsufficient hardness is supplied to surface layer portions thereof andformation of grain boundary precipitates is suppressed. Consequently,inner and outer races 31 and 32 and ball 33 serving as the mechanicalcomponents in the present exemplary variation are made of steelcontaining at least 3.75 mass % of chromium and provided withnitrogen-enriched layers on the surface layer portions thereof, whilesufficiently ensuring fatigue resistance and toughness. Furthermore,inner and outer races 31 and 32 and ball 33 allow a universal jointimplemented as constant velocity joint 3 to have a long life.

A method of producing a mechanical component, and a rolling bearing, aconstant velocity joint and the like mechanical element including themechanical component according to the first embodiment will now bedescribed.

Referring to FIG. 10, first, steel members made of steel containing atleast 0.77 mass % and not more than 0.85 mass % of carbon, at least 0.01mass % and not more than 0.25 mass % of silicon, at least 0.01 mass %and not more than 0.35 mass % of manganese, at least 0.01 mass % and notmore than 0.15 mass % of nickel, at least 3.75 mass % and not more than4.25 mass % of chromium, at least 4 mass % and not more than 4.5 mass %of molybdenum and at least 0.9 mass % and not more than 1.1 mass % ofvanadium with a remainder consisting of iron and impurity, and formedinto substantial shapes of mechanical components are prepared, i.e., asteel member preparing step is carried out. More specifically, workingoperations such as cutting, forging and turning are performed on steelbars or steel wires containing the aforementioned components, therebypreparing steel members formed into substantial shapes of the mechanicalcomponents such as outer ring 11, bearing washer 21, inner race 31 andthe like as the mechanical components.

Then, a heat treatment step is carried out by performing a heattreatment including quenching and nitriding the aforementioned steelmembers prepared in the steel member preparing step. The details of thisheat treatment step will be described later.

Then, a finishing step performing finishing etc. on the steel memberssubjected to the heat treatment step is carried out. More specifically,inner ring raceway surface 12A, bearing washer raceway surface 21A,outer race ball groove 32A and the like of the steel members subjectedto the heat treatment step are polished, for example. This completes amechanical component in the present embodiment and the method ofproducing the mechanical component in the present embodiment iscompleted.

Furthermore, the assembling step of assembling the completed mechanicalcomponents into a mechanical element is carried out. More specifically,the mechanical components of the present invention produced through theaforementioned steps, i.e., outer ring 11, inner ring 12, balls 13 andcage 14, for example, are assembled into deep groove ball bearing 1.Thus, a mechanical element including a mechanical component of thepresent invention is produced.

The details of the aforementioned heat treatment step will now bedescribed with reference to FIG. 11. Referring to FIG. 11, time shown inthe horizontal direction elapses rightward. Referring to FIG. 11,further, temperature shown in the vertical direction is increasedupward.

Referring to FIG. 11, the quenching step of quenching the steel membersas the objects to be treated is first carried out in the heat treatmentstep of the method of producing a mechanical component according to thepresent embodiment. More specifically, the steel members are heated to atemperature T₁ equal to or higher than a transformation temperature A₁in a decompressed atmosphere (vacuum) or a salt bath, maintained at thistemperature for a time t₁, and thereafter cooled from the temperature T₁equal to or higher than the transformation temperature A₁ to atemperature equal to or lower than a point M₈, to be quenched.

The transformation temperature A₁ denotes a point corresponding to atemperature at which the structure of the steel starts transforming fromferrite into austenite when the same is heated. The point M_(s) denotesa point corresponding to a temperature at which the steel havingtransformed into austenite starts transforming into martensite when thesame is cooled.

Then, the first tempering step is carried out for tempering the steelmembers subjected to the quenching. More specifically, the steel membersare heated to a temperature T₂ less than the transformation temperatureA₁ in a vacuum, maintained at this temperature for a time t₂, andthereafter cooled, to be tempered, for example. Thus, residual stressresulting from the quenching of the steel members is relaxed, and strainresulting from the heat treatment is suppressed.

Then, the subzero step is carried out on the steel members subjected tothe first tempering step. More specifically, a subzero treatment isperformed by spraying liquid nitrogen, for example, onto the steelmembers for cooling the steel members to a temperature T₃ less than 0°C. and maintaining the same at this temperature for a time t₃.

Thus, retained austenite formed by the quenching of the steel memberstransforms into martensite, for stabilizing the structure of the steel.

Then, the second tempering step is carried out on the steel memberssubjected to the subzero step. More specifically, the steel members areheated to a temperature T₄ less than the transformation temperature A₁in a vacuum, maintained at this temperature for a time t₄, andthereafter cooled, to be tempered, for example. Thus, residual stressresulting from the subzero treatment of the steel members is relaxed,and strain is suppressed.

Then, the steel members subjected to the second tempering step aretempered again through the third tempering step. More specifically, thesteel members are heated to a temperature T₅ less than thetransformation temperature A₁ in a vacuum, maintained at thistemperature for a time t₅, and thereafter cooled, to be temperedsimilarly to the aforementioned second tempering step, for example. Thetemperature T₅ and the time t₅ can be set similarly to the temperatureT₄ and the time t₄ in the second tempering step respectively. Thus,residual stress resulting from the subzero treatment of the steelmembers is relaxed and strain is suppressed, similarly to the secondtempering step. The second and third tempering steps may be carried outas a single step.

Then, the plasma nitriding step is carried out on the steel memberssubjected to the third tempering step. More specifically, the steelmembers are inserted into a plasma nitriding furnace into which nitrogen(N₂) and at least one element selected from the group consisting ofhydrogen (H₂), methane (CH₄) and argon (Ar) are introduced so that thepressure is at least 50 Pa and not more than 5000 Pa, and the steelmembers are heated to a temperature T₆ under conditions of a dischargevoltage of at least 50 V and not more than 1000 V and a dischargecurrent of at least 0.001 A and not more than 100 A, maintained at thistemperature for a time t₆, and thereafter cooled, to be plasma-nitrided,for example. Thus, nitrogen penetrates into the surface layer portionsof the steel members to form nitrogen-enriched layers, thereby improvingthe strength of the surface layer portions. The temperature T₆ can beset to at least 300° C. and not more than 550° C., for example, and thetime t₆ can be set to at least one hour and not more than 80 hours. Theheat treatment conditions such as the temperature T₆ and the time t₆ canbe so decided that grain boundary precipitate layers formed in theplasma nitriding treatment have such thicknesses that the grain boundaryprecipitate layers can be removed in the finishing step in considerationof removal amounts in the finishing performed in the finishing step.

When the steel constituting the steel members is AMS 6490 (AISI M50),the pressure, the discharge voltage, the discharge current, thetemperature T₆ and the time t₆ in the plasma nitriding step arepreferably set to at least 50 Pa and not more than 1000 Pa, at least 50V and not more than 600 V, at least 0.001 A and not more than 300 A, atleast 350° C. and not more than 450° C. and at least one hour and notmore than 50 hours respectively.

Then, the diffusion step is carried out on the steel members subjectedto the plasma nitriding step. More specifically, the steel members areheated to a temperature T₇ in a vacuum and maintained at thistemperature for a time t₇ to be diffusion-treated, for example. Thetemperature T₇ can be set to at least 300° C. and not more than 480° C.,preferably at least 300° C. and not more than 430° C., and the time t₇can be set to at least 50 hours and not more than 300 hours. Thus, thenitrogen having penetrated into the steel can be made to reach desiredregions while suppressing cancellation of increase in the hardness ofthe surface layer portions resulting from formation of nitrided layers.The diffusions step is so carried out that the nitrogen havingpenetrated into the steel can be made to reach the desired regions evenif the depths of the penetration of the nitrogen in the plasma nitridingstep are kept in the range allowing removal of the grain boundaryprecipitate layers in the finishing. The heat treatment step in thepresent embodiment is completed through these steps.

According to the heat treatment method for steel in the presentembodiment, as hereinabove described, hard nitrogen-enriched layers canbe formed by nitriding the surface layer portions of the steelcontaining at least 3.75 mass % of chromium, and formation of grainboundary precipitates can be suppressed by the diffusion step.

According to the method of producing a mechanical component in the aboveembodiment, mechanical components (outer ring 11, bearing washer 21,inner race 31 etc.) made of steel containing at least 3.75 mass % ofchromium, provided with hard nitrogen-enriched layers by nitridingsurface layer portions and inhibited from formation of grain boundaryprecipitates can be produced. Consequently, the nitrogen-enriched layershaving the nitrogen concentrations of at least 0.05 mass %, the totalsof the carbon concentrations and the nitrogen concentrations of at least0.82 mass % and not more than 1.9 mass %, the thicknesses of at least0.11 mm and the hardness of at least 830 HV are formed on the regionsincluding the surfaces (outer ring raceway surface 11A, bearing washerraceway surface 21A, a surface of inner race ball groove 31A etc.) ofthe mechanical components (outer ring 11, bearing washer 21, inner race31 etc.) in the present embodiment as hereinabove described, while thenumber of detected grain boundary precipitates can be reduced to notmore than one when each nitrogen-enriched layer is cut along a crosssection perpendicular to the surface thereof and the cross section isobserved with an optical microscope or an SEM randomly in five fields ofview each of a square region including the surface and having each sideof 150 μm. The carbon concentration and the nitrogen concentration ineach nitrogen-enriched layer can be controlled by adjusting thetreatment time of the plasma nitriding performed in the plasma nitridingstep and the treating time of the diffusion treatment performed in thediffusion step, for example.

Second Embodiment

With reference to FIG. 12, the present invention in a second embodimentprovides a turbo fan engine configured, as will be describedhereinafter.

Referring to FIG. 12, a turbofan engine 70 includes a compressionportion 71, a combustion portion 72 and a turbine portion 73. Turbofanengine 70 further includes a low-pressure main shaft 74 so arranged asto reach turbine portion 73 from compression portion 71 throughcombustion portion 72 and a high-pressure main shaft 77 so arranged asto enclose the outer circumferential surface of low-pressure main shaft74.

Compression portion 71 includes a fan 75 having a plurality of fanblades 75A connected to low-pressure main shaft 74 and so formed as toradially outwardly protrude from low-pressure main shaft 74, a fannacelle 76 enclosing the outer peripheral side of fan 75 and extendingtoward combustion portion 72 and a compressor 81 arranged on the sidecloser to combustion portion 72 as viewed from fan 75. Compressor 81 hasa low-pressure compressor 81A and a high-pressure compressor 81Barranged on the side closer to combustion portion 72 as viewed fromlow-pressure compressor 81A. Low-pressure compressor 81A has a pluralityof compressor blades 88 connected to low-pressure main shaft 74 toradially outwardly protrude from low-pressure main shaft 74 and arrangedin line in a direction for approaching combustion portion 72 from theside of fan 75. High-pressure compressor 81B also has a plurality ofcompressor blades 88 connected to high-pressure main shaft 77 toradially outwardly protrude from high-pressure main shaft 77 andarranged in line in the direction for approaching combustion portion 72from the side of fan 75. Further, a core cowl 78 is so arranged as toenclose the outer peripheral side of low-pressure compressor 81A. Anannular space between core cowl 78 and fan nacelle 76 constitutes abypass passage 79.

Combustion portion 72 includes a combustion chamber 82 connected tohigh-pressure compressor 81B of compression portion 71 and provided witha fuel supply member and an ignition member (not shown). Turbine portion73 includes a turbine 83 having a high-pressure turbine 83B and alow-pressure turbine 83A arranged on a side opposite to combustionportion 72 as viewed from high-pressure turbine 83B. Further, a turbinenozzle 84 externally discharging combustion gas from turbine 83 isarranged on a side opposite to high-pressure turbine 83B as viewed fromlow-pressure turbine 83A. Low-pressure turbine 83A has a plurality ofturbine blades 87 connected to low-pressure main shaft 74 to radiallyoutwardly protrude from low-pressure main shaft 74 and arranged in linein a direction for approaching turbine nozzle 84 from the side ofcombustion chamber 82. High-pressure turbine 83B also has a plurality ofturbine blades 87 connected to high-pressure main shaft 77 to radiallyoutwardly protrude from high-pressure main shaft 77 and arranged in linein the direction for approaching turbine nozzle 84 from the side ofcombustion chamber 82.

Low-pressure main shaft 74 and high-pressure main shaft 77 as rotatingmembers which are main shafts or members rotating upon rotation of themain shafts are supported by rolling bearings 89 to be rotatable withrespect to members arranged adjacently to low-pressure main shaft 74 andhigh-pressure main shaft 77. In other words, rolling bearing 89 isprovided in turbofan engine 70 serving as a gas turbine engine andsupports low-pressure main shaft 74 or high-pressure main shaft 77 asthe rotating member which is the main shaft or the member rotating uponrotation of the main shaft to be rotatable with respect to the memberadjacent to low-pressure main shaft 74 or high-pressure main shaft 77.

Operations of turbofan engine 70 according to the present embodimentwill now be described. Referring to FIG. 12, air on the side opposite tocombustion portion 72 as viewed from fan 75, i.e., on the front side ofturbofan engine 70 is incorporated into the space enclosed with fannacelle 76 by fan 75 rotating around the axis of low-pressure main shaft74 (arrow α). Part of the incorporated air flows along arrow β, and isexternally discharged as an air jet through bypass passage 79. This airjet partially forms a part of the thrust generated by turbofan engine70.

On the other hand, the rest of the air incorporated into the spaceenclosed with fan nacelle 76 flows into compressor 81 along arrow γ. Theair flowing into compressor 81 is compressed by flowing towardhigh-pressure compressor 81B through low-pressure compressor 81A havingthe plurality of compressor blades 88 rotating around the axis oflow-pressure main shaft 74, and flows into high-pressure compressor 81B.The air flowing into high-pressure compressor 81B is further compressedby flowing toward combustion chamber 82 through high-pressure compressor81B having the plurality of compressor blades 88 rotating around theaxis of high-pressure main shaft 77, and flows into combustion chamber82 (arrow δ).

The air compressed in compressor 81 to flow into combustion chamber 82is mixed with a fuel supplied into combustion chamber 82 by the fuelsupply member (not shown), and ignited by the ignition member (notshown). Thus, combustion gas is generated in combustion chamber 82. Thiscombustion gas flows out of combustion chamber 82, to flow into turbine83 (arrow ε).

The combustion gas flowing into turbine 83 collides with turbine blades87 connected to high-pressure main shaft 77 in high-pressure turbine83B, thereby rotating high-pressure main shaft 77 around the axis. Thus,high-pressure compressor 81B having compressor blades 88 connected tohigh-pressure main shaft 77 is driven. Further, the combustion gaspassing through high-pressure turbine 83B collides with turbine blades87 connected to low-pressure main shaft 74 in low-pressure turbine 83A,thereby rotating low-pressure main shaft 74 around the axis. Thus,low-pressure compressor 81A having compressor blades 88 connected tolow-pressure main shaft 74 and fan 75 having fan blades 75A connected tolow-pressure main shaft 74 are driven.

The combustion gas passing through low-pressure turbine 83A isexternally discharged from turbine nozzle 84. The discharged combustiongas jetted forms a part of the thrust generated by turbofan engine 70.

In turbofan engine 70, rolling bearing 89 supporting low-pressure mainshaft 74 or high-pressure main shaft 77 to be rotatable with respect tothe member adjacent to low-pressure main shaft 74 or high-pressure mainshaft 77 is used in a high-temperature environment due to influence byheat generated in turbofan engine 70. Further, hard foreign matter suchas metallic powder or carbon powder may penetrate into rolling bearing89. Therefore, suppression of reduction in hardness of bearingcomponents in the high-temperature environment and improvement indurability in a contaminated environment are required to rolling bearing89. Furthermore, smearing must be reduced/prevented to support rapidrotation of low-pressure main shaft 74 or high-pressure main shaft 77.Further, dry-run performance is also required to rolling bearing 89 sothat, even if lubrication of rolling bearing 89 is temporarily stoppedby some cause when turbofan engine 70 is installed in an aircraft,rolling bearing 89 continuously rotatably supports low-pressure mainshaft 74 or high-pressure main shaft 77 without seizure until thelubrication is recovered.

When rolling bearing 89 is formed by a rolling bearing according to thesecond embodiment of the present invention described below, theaforementioned requirements can be satisfied.

Referring to FIG. 13, a three-point contact ball bearing 4 is basicallysimilar in configuration and effect to deep groove ball bearing 1 in thefirst embodiment. More specifically, three-point contact ball bearing 4includes an annular outer ring 41 serving as a race member, an annularinner ring 42 serving as a race member arranged inside outer ring 41,and a plurality of balls 43 as rolling elements arranged between outerring 41 and inner ring 42 and retained in an annular cage 44. An outerring raceway surface 41A is formed on the inner circumferential surfaceof outer ring 41, while an inner ring raceway surface 42A is formed onthe outer circumferential surface of inner ring 42. Outer ring 41 andinner ring 42 are so arranged that inner ring raceway surface 42A andouter ring raceway surface 41A are opposed to each other. Inner ring 42includes a first inner ring 421 and a second inner ring 422, and isdivided along the axial center. A first inner ring raceway surface 421Aand a second inner ring raceway surface 422A are formed on the outercircumferential surfaces of first inner ring 421 and second inner ring422 respectively. First inner ring raceway surface 421A and second innerring raceway surface 422A constitute inner ring raceway surface 42A. Theplurality of balls 43 are contactable with first inner ring racewaysurface 421A, second inner ring raceway surface 422A and outer ringraceway surface 41A on ball rolling contact surfaces 43A which are theouter circumferential surfaces thereof, and arranged at a prescribedpitch circumferentially by cage 44, to be retained on an annular racewayin a rollable manner. Outer ring 41 and inner ring 42 of three-pointcontact ball bearing 4 are mutually relatively rotatable due to theaforementioned structure.

Outer ring 41 and inner ring 42 serving as the race members are made ofsteel containing at least 0.77 mass % and not more than 0.85 mass % ofcarbon, at least 0.01 mass % and not more than 0.25 mass % of silicon,at least 0.01 mass % and not more than 0.35 mass % of manganese, atleast 0.01 mass % and not more than 0.15 mass % of nickel, at least 3.75mass % and not more than 4.25 mass % of chromium, at least 4 mass % andnot more than 4.5 mass % of molybdenum and at least 0.9 mass % and notmore than 1.1 mass % of vanadium with a remainder consisting of iron andimpurity. Referring to FIG. 13, an outer ring nitrogen-enriched layer41B and an inner ring nitrogen-enriched layer 42B having nitrogenconcentrations of at least 0.05 mass % are formed on regions includingouter ring raceway surface 41A and inner ring raceway surface 42A whichare the surfaces of outer ring 41 and inner ring 42. Further, the totalsof carbon concentrations and the nitrogen concentrations in outer ringnitrogen-enriched layer 41B and inner ring nitrogen-enriched layer 42Bare at least 0.82 mass % and not more than 1.9 mass %. Theaforementioned impurity includes unavoidable impurity such as thatderived from the raw materials for the steel, that mixed in productionsteps and the like.

Balls 43 serving as the rolling elements are made of ceramics. Morespecifically, balls 43 are formed by sintered bodies mainly composed ofsilicon nitride with a remainder consisting of impurity in the presentembodiment. The sintered bodies may contain a sintering additive such asaluminum oxide (Al₂O₃) or yttrium oxide (Y₂O₃).

Outer ring 41 and inner ring 42 serving as the race members ofthree-point contact ball bearing 4 according to the present embodimentare made of the steel having the aforementioned proper componentcomposition, and outer ring nitrogen-enriched layer 41B and inner ringnitrogen-enriched layer 42B having the nitrogen concentrations of atleast 0.05 mass % are formed on the regions including outer ring racewaysurface 41A and inner ring raceway surface 42A formed on the surfacesthereof. The totals of the carbon concentrations and the nitrogenconcentrations in outer ring nitrogen-enriched layer 41B and inner ringnitrogen-enriched layer 42B are so set in the proper range of at least0.82 mass % and not more than 1.9 mass % that sufficient hardness issupplied to surface layer portions thereof and formation of grainboundary precipitates is suppressed. Consequently, outer ring 41 andinner ring 42 serving as the race members in the present embodiment aremade of steel containing at least 3.75 mass % of chromium and providedwith nitrogen-enriched layers on the surface layer portions thereof,while sufficiently ensuring fatigue resistance and toughness.

In three-point contact ball bearing 4 according to the presentembodiment, balls 43 serving as the rolling elements are made ofceramics. Thus, smearing is reduced/prevented, and furthermore, as outerand inner rings 41 and 42 and balls 43 coming into contact with eachother are made of different materials, the seizure resistance isimproved. Consequently, smearing resistance, and simultaneously,endurance in an insufficiently lubricated environment, such as dry-runperformance, are improved. The ceramics having higher hardness than thesteel is employed as the material for balls 43, whereby durability ofballs 43 is improved in a contaminated environment. Further, balls 43are so made of the ceramics that reduction in hardness of balls 43 in ahigh-temperature environment is suppressed. In addition, balls 43 are somade of the ceramics that the weights of balls 43 as well as centrifugalforce acting on balls 43 are reduced as compared with a case where balls43 are made of steel, whereby three-point contact ball bearing 4 issuitable as a rolling bearing supporting a member rotating at a highspeed, in particular.

In three-point contact ball bearing 4 according to the presentembodiment, as hereinabove described, outer ring 41 and inner ring 42serving as the race members are made of the steel containing at least3.75 mass % of chromium, and ball 43 serving as the rolling element ismade of ceramic, whereby reduction in hardness of the bearing componentsin a high-temperature environment is suppressed. Further, outer ringnitrogen-enriched layer 41B and inner ring nitrogen-enriched layer 42Bin which the totals of the carbon concentrations and the nitrogenconcentrations are set in the proper range are formed on the regionsincluding outer ring raceway surface 41A and inner ring raceway surface42A of outer ring 41 and inner ring 42 made of the steel having theproper component composition, and ball 43 is made of ceramic, wherebythe durability of the bearing components in a contaminated environmentis improved. In addition, outer and inner rings 41 and 42 of steel incombination with ceramic ball 43 achieve improved smearing resistanceand improved dry-run performance. Consequently, three-point contact ballbearing 4 is a ball bearing attaining not only suppression of reductionin hardness of the bearing components in a high-temperature environment,improvement in durability in a contaminated environment, improvement insmearing resistance, but also improvement of the dry-run performance.

With reference to FIG. 15 and FIG. 16, the present embodiment in anexemplary variation provides a cylindrical roller bearing, as will bedescribed hereinafter.

Referring to FIG. 15, a cylindrical roller bearing 5 includes an annularouter ring 51, an annular inner ring 52 arranged inside outer ring 51and a plurality of rollers 53 as rolling elements arranged between outerring 51 and inner ring 52 and retained in an annular cage 54. Rollers 53are cylindrical. An outer ring raceway surface 51A is formed on theinner circumferential surface of outer ring 51, while an inner ringraceway surface 52A is formed on the outer circumferential surface ofinner ring 52. Outer ring 51 and inner ring 52 are so arranged thatinner ring raceway surface 52A and outer ring raceway surface 51A areopposed to each other. The plurality of rollers 53 are in contact withinner ring raceway surface 52A and outer ring raceway surface 51A onroller rolling contact surfaces 53A which are the outer circumferentialsurfaces thereof, and arranged at a prescribed pitch circumferentiallyby cage 54, to be retained on an annular raceway in a rollable manner.Outer ring 51 and inner ring 52 of cylindrical roller bearing 5 aremutually relatively rotatable due to the aforementioned structure.

Referring to FIG. 16, outer ring 51, inner ring 52 and rollers 53 ofcylindrical roller bearing 5 in the present exemplary variationcorrespond to outer ring 41, inner ring 42 and balls 43 of three-pointcontact ball bearing 4, as described above, respectively, have similarstructures, and exhibit similar effects. In other words, a race memberimplemented as outer ring 51 and inner ring 52 is made of steelcontaining at least 0.77 mass % and not more than 0.85 mass % of carbon,at least 0.01 mass % and not more than 0.25 mass % of silicon, at least0.01 mass % and not more than 0.35 mass % of manganese, at least 0.01mass % and not more than 0.15 mass % of nickel, at least 3.75 mass % andnot more than 4.25 mass % of chromium, at least 4 mass % and not morethan 4.5 mass % of molybdenum and at least 0.9 mass % and not more than1.1 mass % of vanadium with a remainder consisting of iron and impurity.

Furthermore, referring to FIG. 16, an outer ring nitrogen-enriched layer51B and an inner ring nitrogen-enriched layer 52B having nitrogenconcentrations of at least 0.05 mass % are formed on regions includingouter ring raceway surface 51A and inner ring raceway surface 52A whichare the surfaces of outer ring 51 and inner ring 52 respectively. Thetotals of carbon concentrations and the nitrogen concentrations in outerring nitrogen-enriched layer 51B and inner ring nitrogen-enriched layer52B are at least 0.82 mass % and not more than 1.9 mass %.

Furthermore, a rolling element implemented as roller 53 is of ceramic,such as a sintered body mainly composed of silicon nitride.

In the present exemplary variation, cylindrical roller bearing 5, aswell as three-point contact ball bearing 4, has outer ring 51 and innerring 52, serving as race members, made of steel containing at least 3.75mass % of chromium, and roller 53, serving as a rolling element, made ofceramic, whereby reduction in hardness of the bearing components in ahigh-temperature environment is suppressed. Further, outer ringnitrogen-enriched layer 51B and inner ring nitrogen-enriched layer 52Bin which the totals of the carbon concentrations and the nitrogenconcentrations are set in the proper range are formed on the regionsincluding outer ring raceway surface 51A and inner ring raceway surface52A of outer ring 51 and inner ring 52 made of the steel having theproper component composition, and roller 53 is made of ceramic, wherebythe durability of the bearing components in a contaminated environmentis improved. In addition, outer and inner rings 51 and 52 of steel incombination with ceramic roller 53 achieve improved smearing resistanceand improved dry-run performance. Consequently, cylindrical rollerbearing 5 is a rolling bearing attaining not only suppression ofreduction in hardness of the bearing components in a high-temperatureenvironment, improvement in durability in a contaminated environment,improvement in smearing resistance, but also improvement of the dry-runperformance.

The method of producing a rolling bearing according to the secondembodiment of the present invention will now be described.

With reference to FIG. 17, the present embodiment provides the rollingbearing, produced in a method including a race member preparing stepincluding steps (S10) to (S90), a rolling element preparing stepincluding steps (S110) to (S150), and an assembling step carried out asa step (S200).

First, the race member preparing step will be described. In a steelmember preparing step carried out as the step (S10), steel members madeof steel containing at least 0.77 mass % and not more than 0.85 mass %of carbon, at least 0.01 mass % and not more than 0.25 mass % ofsilicon, at least 0.01 mass % and not more than 0.35 mass % ofmanganese, at least 0.01 mass % and not more than 0.15 mass % of nickel,at least 3.75 mass % and not more than 4.25 mass % of chromium, at least4 mass % and not more than 4.5 mass % of molybdenum and at least 0.9mass % and not more than 1.1 mass % of vanadium with a remainderconsisting of iron and impurity, and formed into substantial shapes ofrace members are prepared. More specifically, working operations such ascutting, forging and turning are performed on steel bars or steel wirescontaining the aforementioned components, thereby preparing steelmembers formed into substantial shapes of outer rings 41, 51 and innerrings 42, 52 as the race members.

Then, a heat treatment step is carried out by performing a heattreatment including quenching and nitriding on the aforementioned steelmembers prepared in the step (S10). The heat treatment step includes aquenching step carried out as the step (S20), a first tempering stepcarried out as the step (S30), a subzero step carried out as the step(S40), a second tempering step carried out as the step (S50), a thirdtempering step carried out as the step (S60), a plasma nitriding stepcarried out as the step (S70) and a diffusion step carried out as thestep (S80). This heat treatment step can be performed similarly as donein the first embodiment.

Then, a finishing step performing finishing etc. on the steel memberssubjected to the heat treatment step is carried out as the step (S90).More specifically, outer ring raceway surfaces 41A, 51A and inner ringraceway surfaces 42A, 52A of the steel members subjected to the heattreatment step are polished, for example. Thus, the race members in thepresent embodiment are completed, and the race member preparing step inthe present embodiment is completed.

According to the race member preparing step in the above embodiment,race members (outer rings 41, 51 and inner rings 42, 52 etc.) made ofsteel containing at least 3.75 mass % of chromium, provided with hardnitrogen-enriched layers by nitriding surface layer portions andinhibited from formation of grain boundary precipitates can be produced.Consequently, the nitrogen-enriched layers having the nitrogenconcentrations of at least 0.05 mass %, the totals of the carbonconcentrations and the nitrogen concentrations of at least 0.82 mass %and not more than 1.9 mass %, the thicknesses of at least 0.1 mm and thehardness of at least 830 HV are formed on the regions including thesurfaces (outer ring raceway surfaces 41A, 51A and inner ring racewaysurfaces 42A, 52A etc.) of the race members (outer rings 41, 51 andinner rings 42, 52 etc.) in the present embodiment, while the number ofdetected grain boundary precipitates can be reduced to not more than onewhen each nitrogen-enriched layer is cut along a cross sectionperpendicular to the surface thereof and the cross section is observedwith an optical microscope or an SEM randomly in five fields of vieweach of a square region including the surface and having each side of150 μm.

Referring to FIG. 17, the rolling element preparing step is performed asfollows: initially, step (S110) is performed to prepare ceramic powder,i.e., a powder preparing step is performed. More specifically, powder ofceramics, such as silicon nitride, for example, employed as a materialconstituting the rolling elements is prepared. Then, step (S120), or amixing step, is performed. More specifically, a sintering additive isadded to and mixed with the ceramic powder prepared in step (S110).

Then, referring to FIG. 17, the mixture of the ceramic powder and thesintering additive is formed into substantial shapes of rollingelements, i.e., step (S130) is performed. More specifically, the mixtureof the ceramic powder and the sintering additive is molded by pressmolding, casting, extrusion molding or rolling granulation, therebypreparing formed bodies formed into substantial shapes of balls 43,rollers 53 serving as the rolling elements.

Then, as step (S140), these formed bodies are sintered. Morespecifically, the aforementioned formed bodies are sintered by pressuresintering such as HIP (hot isostatic pressing) or GPS (gas pressuresintering), to obtain sintered bodies having substantial shapes of outerrings 41, 51 and inner rings 42, 52.

Then, the surfaces of the sintered bodies obtained in step (S140) areworked and regions including the surfaces are removed, i.e., a finishingis performed to complete the rolling contact member, i.e., a finishingstep is performed as step (S150). More specifically, the surfaces (orrolling contact surfaces) of the sintered bodies obtained in thesintering step are polished to complete balls 43, rollers 53 as therolling elements. Thus the rolling element preparing step in the presentembodiment is completed.

Then, referring to FIG. 17, the assembling step of assembling thecompleted bearing components into a rolling bearing is carried out asthe step (S200). More specifically, outer ring 41, inner ring 42 andballs 43 prepared through the aforementioned steps and cage 44separately prepared are assembled into three-point contact ball bearing4, for example. Thus, the rolling bearing according to the presentembodiment is completed.

Third Embodiment

The present invention in a third embodiment will now be described. Amechanical component according to the third embodiment is basicallysimilar in structure to the case of the first embodiment, and can besimilarly produced. However, the third embodiment is different from thefirst embodiment in the component composition of steel serving as amaterial and a heat treatment method, as described below.

Referring to FIG. 1 and FIG. 2, outer ring 11, inner ring 12, and ball13 serving as mechanical components in the third embodiment areconstituted of steel containing at least 0.11 mass % and not more than0.15 mass % of carbon, at least 0.1 mass % and not more than 0.25 mass %of silicon, at least 0.15 mass % and not more than 0.35 mass % ofmanganese, at least 3.2 mass % and not more than 3.6 mass % of nickel,at least 4 mass % and not more than 4.25 mass % of chromium, at least 4mass % and not more than 4.5 mass % of molybdenum and at least 1.13 mass% and not more than 1.33 mass % of vanadium with a remainder consistingof iron and impurity. Referring to FIG. 2, outer ring nitrogen-enrichedlayer 11B, inner ring nitrogen-enriched layer 12B and ballnitrogen-enriched layer 13B having nitrogen concentrations of at least0.05 mass % are formed on regions including outer ring raceway surface11A, inner ring raceway surface 12A and ball rolling contact surface 13Awhich are the surfaces of outer ring 11, inner ring 12, and ball 13. Thetotals of carbon concentrations and the nitrogen concentrations in outerring nitrogen-enriched layer 11B, inner ring nitrogen-enriched layer 12Band ball nitrogen-enriched layer 13B are at least 0.55 mass % and notmore than 1.9 mass %. The aforementioned impurity includes unavoidableimpurity such as that derived from the raw materials for the steel, thatmixed in production steps and the like.

Outer ring 11, inner ring 12 and ball 13 serving as the mechanicalcomponents according to the present embodiment are made of the steelhaving the aforementioned proper component composition, and outer ringnitrogen-enriched layer 11B, inner ring nitrogen-enriched layer 12B andball nitrogen-enriched layer 13B having the nitrogen concentrations ofat least 0.05 mass % are formed on the regions including outer ringraceway surface 11A, inner ring raceway surface 12A and ball rollingcontact surface 13A formed on the surfaces thereof. The totals of thecarbon concentrations and the nitrogen concentrations in outer ringnitrogen-enriched layer 11B, inner ring nitrogen-enriched layer 12B andball nitrogen-enriched layer 13B are so set in the proper range of atleast 0.55 mass % and not more than 1.9 mass % that sufficient hardnessis supplied to surface layer portions thereof and formation of grainboundary precipitates is suppressed. Consequently, outer ring 11, innerring 12 and ball 13 serving as the mechanical components in the presentembodiment are made of steel containing at least 4 mass % of chromiumand provided with nitrogen-enriched layers on the surface layer portionsthereof, while sufficiently ensuring fatigue resistance and toughness.Furthermore, outer ring 11, inner ring 12 and ball 13 allow a rollingbearing implemented as deep groove ball bearing 1 to have a long life.

Preferably, the thicknesses of outer ring nitrogen-enriched layer 11B,inner ring nitrogen-enriched layer 12B and ball nitrogen-enriched layer13B formed on outer ring 11, inner ring 12 and ball 13 are at least 0.11mm. Thus, sufficient strength is supplied to outer ring 11, inner ring12 and ball 13.

Preferably, outer ring nitrogen-enriched layer 11B, inner ringnitrogen-enriched layer 12B and ball nitrogen-enriched layer 13B havehardness of at least 800 HV. Thus, the strength of outer ring 11, innerring 12 and ball 13 can be more reliably ensured.

Preferably, the number of nitrides of iron each having an aspect ratioof at least 2 and a length of at least 7.5 μm is not more than one infive fields of view of square regions of 150 μm on each side when outerring nitrogen-enriched layer 11B, inner ring nitrogen-enriched layer 12Band ball nitrogen-enriched layer 13B are observed with a microscope.Thus, the fatigue resistance of outer ring 11, inner ring 12 and ball 13can be improved.

Furthermore, with reference to FIG. 3 and FIG. 4, the present embodimentin the first exemplary variation, i.e., bearing washer 21 and needleroller 23 of thrust needle roller bearing 2 correspond to outer andinner rings 11 and 12 and ball 13 of deep groove ball bearing 1,respectively, in the above-described present embodiment, and the formeris similar to the latter in configuration and effect. More specifically,bearing washer 21 and needle roller 23 serving as the mechanicalcomponents are made of steel containing at least 0.11 mass % and notmore than 0.15 mass % of carbon, at least 0.1 mass % and not more than0.25 mass % of silicon, at least 0.15 mass % and not more than 0.35 mass% of manganese, at least 3.2 mass % and not more than 3.6 mass % ofnickel, at least 4 mass % and not more than 4.25 mass % of chromium, atleast 4 mass % and not more than 4.5 mass % of molybdenum and at least1.13 mass % and not more than 1.33 mass % of vanadium with a remainderconsisting of iron and impurity. Referring to FIG. 4, bearing washernitrogen-enriched layer 21B and roller nitrogen-enriched layer 23Bhaving nitrogen concentrations of at least 0.05 mass % are formed onregions including bearing washer raceway surface 21A and roller rollingcontact surface 23A which are the surfaces of bearing washer 21 andneedle roller 23. The totals of carbon concentrations and the nitrogenconcentrations in bearing washer nitrogen-enriched layer 21B and rollernitrogen-enriched layer 23B are at least 0.55 mass % and not more than1.9 mass %.

Bearing washer 21 and needle roller 23 serving as the mechanicalcomponents according to the present exemplary variation are made ofsteel having the aforementioned proper component composition, andprovided with bearing washer nitrogen-enriched layer 21B and rollernitrogen-enriched layer 23B having nitrogen concentrations of at least0.05 mass % on regions including bearing washer raceway surface 21A androller rolling contact surface 23A formed on the surfaces thereof. Thetotals of carbon concentrations and the nitrogen concentrations inbearing washer nitrogen-enriched layer 21B and a rollernitrogen-enriched layer 23B are set in the proper range of at least 0.55mass % and not more than 1.9 mass %, so that sufficient hardness issupplied to surface layer portions and formation of grain boundaryprecipitates is suppressed. Consequently, bearing washer 21 and needleroller 23 serving as the mechanical components in the present exemplaryvariation are mechanical components made of steel containing at least 4mass % of chromium and provided with the nitrogen-enriched layers on thesurface layer portions thereof, while sufficiently ensuring fatigueresistance and toughness. Furthermore, bearing washer 21 and needleroller 23 allow the rolling bearing, or thrust needle roller bearing 2,to have a long life.

Furthermore, with reference to FIG. 5 to FIG. 9, the present embodimentin the second exemplary variation, i.e., the constant velocity joint 3inner and outer races 31 and 32 and ball 33 correspond to the deepgroove ball bearing 1 outer and inner rings 11 and 12 and ball 13,respectively, in the above-described present embodiment, and the formeris similar to the latter in configuration and effect. More specifically,inner and outer races 31 and 32 and ball 33 serving as the mechanicalcomponents are made of steel containing at least 0.11 mass % and notmore than 0.15 mass % of carbon, at least 0.1 mass % and not more than0.25 mass % of silicon, at least 0.15 mass % and not more than 0.35 mass% of manganese, at least 3.2 mass % and not more than 3.6 mass % ofnickel, at least 4 mass % and not more than 4.25 mass % of chromium, atleast 4 mass % and not more than 4.5 mass % of molybdenum and at least1.13 mass % and not more than 1.33 mass % of vanadium with a remainderconsisting of iron and impurity. Referring to FIG. 8 and FIG. 9, innerrace nitrogen-enriched layer 31B, outer race nitrogen-enriched layer32B, and ball nitrogen-enriched layer 33B having nitrogen concentrationsof at least 0.05 mass % are formed on regions including a surface ofinner race ball groove 31A, a surface of outer race ball groove 32A, andball rolling contact surface 33A which are the surfaces of inner andouter races 31 and 32 and ball 33. The totals of carbon concentrationsand the nitrogen concentrations in inner race nitrogen-enriched layer31B, outer race nitrogen-enriched layer 32B, and ball nitrogen-enrichedlayer 33B are at least 0.55 mass % and not more than 1.9 mass %.

Inner and outer races 31 and 32 and ball 33 serving as the mechanicalcomponents according to the present exemplary variation are made ofsteel having the aforementioned proper component composition, andprovided with inner race nitrogen-enriched layer 31B, outer racenitrogen-enriched layer 32B, and ball nitrogen-enriched layer 33B havingnitrogen concentrations of at least 0.05 mass % on regions including thesurface of inner race ball groove 31A, the surface of outer race ballgroove 32A, and ball rolling contact surface 33A formed on the surfacesthereof. The totals of carbon concentrations and the nitrogenconcentrations in inner race nitrogen-enriched layer 31B, outer racenitrogen-enriched layer 32B, and ball nitrogen-enriched layer 33B areset in the proper range of at least 0.55 mass % and not more than 1.9mass %, so that sufficient hardness is supplied to surface layerportions and formation of grain boundary precipitates is suppressed.Consequently, inner and outer races 31 and 32 and ball 33 serving as themechanical components in the present exemplary variation are mechanicalcomponents made of steel containing at least 4 mass % of chromium andprovided with the nitrogen-enriched layers on the surface layer portionsthereof, while sufficiently ensuring fatigue resistance and toughness.Furthermore, inner race 31, outer race 32 and ball 33 allow theuniversal joint, or constant velocity joint 3, to have a long life.

Hereinafter a method of producing a mechanical component, and a rollingbearing, a constant velocity joint and the like mechanical elementincluding the mechanical component according to the third embodimentwill now be described.

With reference to FIG. 18, initially, steel members made of steelcontaining at least 0.11 mass % and not more than 0.15 mass % of carbon,at least 0.1 mass % and not more than 0.25 mass % of silicon, at least0.15 mass % and not more than 0.35 mass % of manganese, at least 3.2mass % and not more than 3.6 mass % of nickel, at least 4 mass % and notmore than 4.25 mass % of chromium, at least 4 mass % and not more than4.5 mass % of molybdenum and at least 1.13 mass % and not more than 1.33mass % of vanadium with a remainder consisting of iron and impurity, andformed into substantial shapes of mechanical components are prepared,i.e., a steel member preparing step is carried out. More specifically,working operations such as cutting, forging and turning are performed onsteel bars or steel wires containing the aforementioned components,thereby preparing steel members formed into substantial shapes of themechanical components such as outer ring 11, bearing washer 21, innerrace 31 and the like as the mechanical components.

Then, a heat treatment step is carried out by performing a heattreatment including quenching and nitriding the aforementioned steelmembers prepared in the steel member preparing step. The details of thisheat treatment step will be described later.

Then, a finishing step performing finishing etc. on the steel memberssubjected to the heat treatment step is carried out. More specifically,inner ring raceway surface 12A, bearing washer raceway surface 21A,outer race ball groove 32A and the like of the steel members subjectedto the heat treatment step are polished, for example. This completes amechanical component in the present embodiment and the method ofproducing the mechanical component in the present embodiment iscompleted.

Furthermore, the assembling step of assembling the completed mechanicalcomponents into a mechanical element is carried out. More specifically,the mechanical components of the present invention produced through theaforementioned steps, e.g., outer ring 11, inner ring 12, ball 13 andcage 14 are assembled into deep groove ball bearing 1. Thus, amechanical element including a mechanical component of the presentinvention is produced.

The details of the heat treatment step above will now be described withreference to FIG. 18 and FIG. 19. Referring to FIG. 19, time shown inthe horizontal direction elapses rightward. Referring to FIG. 19,further, temperature shown in the vertical direction is increasedupward.

Referring to FIG. 18, the carburizing step is first carried out forcarburizing the steel members as the objects to be treated in the heattreatment step in the method of producing the mechanical componentaccording to the present embodiment. More specifically, referring toFIG. 19, the steel members are heated to a temperature T₁₁ equal to orhigher than the transformation temperature A₁ in an atmosphere ofcarburizing gas containing carbon monoxide and hydrogen, and maintainedat this temperature for a time t₁₁, for example, so that carbonpenetrates into surface layer portions of the steel members. Thus,carburized layers having higher carbon concentrations as compared withinner regions other than regions including the surfaces of the steelmembers are formed on the regions including the surfaces of the steelmembers.

Referring to FIG. 18, the quenching step is carried out on the steelmembers subjected to the carburization treatment. More specifically,with reference to FIG. 19, the steel members are cooled from thetemperature T₁₁ equal to or higher than the transformation temperatureA₁ to a temperature equal to or lower than the point M_(s), to bequench-hardened.

Then, with reference to FIG. 18, the first tempering step is carried outon the quenched steel members. More specifically, referring to FIG. 19,the steel members are heated to a temperature T₁₂ less than thetransformation temperature A₁ in a decompressed atmosphere (vacuum),maintained at this temperature for a time t₁₂, and thereafter cooled tobe tempered, for example. Thus, residual stress resulting from thequenching of the steel members is relaxed, and strain resulting from theheat treatment is suppressed.

Then, with reference to FIG. 18, the first subzero step is carried outon the steel members subjected to the first tempering step. Morespecifically, referring to FIG. 19, a subzero treatment is performed byspraying liquid nitrogen, for example, onto the steel members forcooling the steel members to a temperature T₁₃ less than 0° C., andmaintaining the same at this temperature for a time t₁₃. Thus, retainedaustenite formed by the quenching of the steel members transforms intomartensite, for stabilizing the structure of the steel.

Then, with reference to FIG. 18, the second tempering step is carriedout on the steel members subjected to the first subzero step. Morespecifically, referring to FIG. 19, for example, the steel members areheated to a temperature T₁₄ less than the transformation temperature A₁in a decompressed atmosphere (vacuum), maintained at this temperaturefor a time t₁₄, and thereafter cooled, to be tempered. Thus, residualstress resulting from the subzero treatment of the steel members isrelaxed, and strain is suppressed.

Then, with reference to FIG. 18, the second subzero step is carried outon the steel members subjected to the second tempering step. Morespecifically, referring to FIG. 19, the subzero treatment is performedagain by spraying liquid nitrogen, for example, onto the steel membersfor cooling the steel members to a temperature T₁₅ less than 0° C., andmaintaining the same at this temperature for a time t₁₅. Thus, retainedaustenite formed by the quenching of the steel members furthertransforms into martensite, for further stabilizing the structure of thesteel.

Then, with reference to FIG. 18, the third tempering step is carried outon the steel members subjected to the second subzero step. Morespecifically, referring to FIG. 19, the steel members are heated to atemperature T₁₆ less than the transformation temperature A₁ in a vacuum,maintained at this temperature for a time t₁₆, and thereafter cooled, tobe tempered, for example, similarly to the aforementioned secondtempering step. The temperature T₁₆ and the time t₁₆ can be setsimilarly to the temperature T₁₄ and the time t₁₄ in the secondtempering step respectively. Thus, residual stress which can result fromthe subzero treatment of the steel members in the second subzero step isrelaxed, and strain is suppressed.

Then, with reference to FIG. 18, the plasma nitriding step is carriedout on the steel members subjected to the third tempering step. Morespecifically, referring to FIG. 19, the steel members are inserted intoa plasma nitriding furnace into which nitrogen (N₂) and at least oneelement selected from the group consisting of hydrogen (H₂), methane(CH₄) and argon (Ar) are introduced so that the pressure is at least 50Pa and not more than 5000 Pa, and the steel members are heated to atemperature T₁₇ under conditions of a discharge voltage of at least 50 Vand not more than 1000 V and a discharge current of at least 0.001 A andnot more than 100 A, maintained at this temperature for a time t₁₇, andthereafter cooled, to be plasma-nitrided, for example. Thus, nitrogenpenetrates into the surface layer portions of the steel members and thusforms a nitrogen enriched layer, thereby improving the strength of thesurface layer portions. The temperature T₁₇ can be set to at least 300°C. and not more than 550° C., for example, and the time t₁₇ can be setto at least one hour and not more than 80 hours. The heat treatmentconditions such as the temperature T₁₇ and the time t₁₇ can be sodecided that grain boundary precipitate layers formed in the plasmanitriding treatment have such thicknesses or smaller that the grainboundary precipitate layers (layers having grain boundary precipitates)can be removed in the finishing step in consideration of removal amountsin the finishing performed in the finishing step.

When the steel constituting the steel members is AMS 6278 (AISI M50NiL), the pressure, the discharge voltage, the discharge current, thetemperature T₁₇ and the time t₁₇ in the plasma nitriding step arepreferably set to at least 50 Pa and not more than 1000 Pa, at least 50V and not more than 600 V, at least 0.001 A and not more than 300 A, atleast 350° C. and not more than 450° C. and at least one hour and notmore than 50 hours respectively.

Then, with reference to FIG. 18, the diffusion step is carried out onthe steel members subjected to the plasma nitriding step. Morespecifically, referring to FIG. 19, the steel members are heated to atemperature T₁₈ in a vacuum and maintained at this temperature for atime t₁₈, to be diffusion-treated, for example. The temperature T₁₈ canbe set to at least 300° C. and not more than 480° C., preferably atleast 300° C. and not more than 430° C., and the time t₁₈ can be set toat least 50 hours and not more than 300 hours. Thus, the nitrogen havingpenetrated into the steel can be made to reach desired regions whilesuppressing cancellation of increase in the hardness of the surfacelayer portions resulting from formation of nitrided layers. Even if aplasma nitriding process is performed to cause nitrogen to penetrateinto steel to a depth falling within a range allowing a finishing stepto remove a grain boundary precipitate layer, the diffusion step allowsthe nitrogen having penetrated into the steel to reach a desired region.The above steps complete the heat treatment step in the presentembodiment.

According to the method of heat treatment for steel in the presentembodiment, as hereinabove described, a nitrogen enriched layer of highhardness can be formed by nitriding a surface layer portion of steelcontaining at least 4 mass % of chromium, and formation of grainboundary precipitates can be suppressed.

Furthermore, according to the method of producing a mechanical componentin the above embodiment, mechanical components (outer ring 11, bearingwasher 21, inner race 31 etc.) made of steel containing at least 4 mass% of chromium, provided with hard nitrogen-enriched layers by nitridingsurface layer portions and inhibited from formation of grain boundaryprecipitates can be produced. Consequently, the nitrogen-enriched layershaving the nitrogen concentrations of at least 0.05 mass %, the totalsof the carbon concentrations and the nitrogen concentrations of at least0.55 mass % and not more than 1.9 mass %, the thicknesses of at least0.11 mm and the hardness of at least 800 HV are formed on the regionsincluding the surfaces (outer ring raceway surface 11A, bearing washerraceway surface 21A, a surface of inner race ball groove 31A etc.) ofthe mechanical components (outer ring 11, bearing washer 21, inner race31 etc.) in the present embodiment as hereinabove described, while thenumber of detected grain boundary precipitates can be reduced to notmore than one when each nitrogen-enriched layer is cut along a crosssection perpendicular to the surface thereof and the cross section isobserved with an optical microscope or an SEM randomly in five fields ofview each of a square region including the surface and having each sideof 150 μm. The carbon concentration and the nitrogen concentration ineach nitrogen-enriched layer can be controlled by adjusting thetreatment time of the plasma nitriding performed in the plasma nitridingstep and the treating time of the diffusion treatment performed in thediffusion step, for example.

Fourth Embodiment

The present invention in a fourth embodiment will now be described. Arolling bearing according to the fourth embodiment is basically similarin structure to the case of the second embodiment, and can be similarlyproduced. However, the fourth embodiment is different from the secondembodiment in the component composition of steel serving as a materialand a heat treatment method, as described below.

More specifically, with reference to FIG. 12 to FIG. 14, in turbofanengine 70, rolling bearing 89 supporting low-pressure main shaft 74 orhigh-pressure main shaft 77 to be rotatable with respect to a memberadjacent to low-pressure main shaft 74 or high-pressure main shaft 77 isused in a high-temperature environment due to influence by heatgenerated in turbofan engine 70. Further, hard foreign matter such asmetallic powder or carbon powder may penetrate into rolling bearing 89.Therefore, suppression of reduction in hardness of bearing components inthe high-temperature environment and improvement in durability in acontaminated environment are required to rolling bearing 89.Furthermore, smearing must be reduced/prevented to support rapidrotation of low-pressure main shaft 74 or high-pressure main shaft 77.Further, dry-run performance is also required to rolling bearing 89 sothat, even if lubrication of rolling bearing 89 is temporarily stoppedby some cause when turbofan engine 70 is installed in an aircraft,rolling bearing 89 continuously rotatably supports low-pressure mainshaft 74 or high-pressure main shaft 77 without seizure until thelubrication is recovered.

When rolling bearing 89 is formed by a rolling bearing according to thefourth embodiment described below, the aforementioned requirements canbe satisfied.

Referring to FIG. 13 and FIG. 14, outer ring 41 and inner ring 42serving as race members are constituted of steel containing at least0.11 mass % and not more than 0.15 mass % of carbon, at least 0.1 mass %and not more than 0.25 mass % of silicon, at least 0.15 mass % and notmore than 0.35 mass % of manganese, at least 3.2 mass % and not morethan 3.6 mass % of nickel, at least 4 mass % and not more than 4.25 mass% of chromium, at least 4 mass % and not more than 4.5 mass % ofmolybdenum and at least 1.13 mass % and not more than 1.33 mass % ofvanadium with a remainder consisting of iron and impurity. Referring toFIG. 2, outer ring nitrogen-enriched layer 41B and inner ringnitrogen-enriched layer 42B having nitrogen concentrations of at least0.05 mass % are formed on regions including outer ring raceway surface41A and inner ring raceway surface 42A which are the surfaces of outerring 41 and inner ring 42. The totals of carbon concentrations and thenitrogen concentrations in outer ring nitrogen-enriched layer 41B andinner ring nitrogen-enriched layer 42B are at least 0.55 mass % and notmore than 1.9 mass %. The aforementioned impurity includes unavoidableimpurity such as that derived from the raw materials for the steel, thatmixed in production steps and the like.

Balls 43 serving as the rolling elements are made of ceramics. Morespecifically, in the present embodiment, balls 43 are formed of sinteredbodies mainly composed of silicon nitride with a remainder consisting ofimpurity. The sintered bodies may contain a sintering additive such asaluminum oxide (Al₂O₃) or yttrium oxide (Y₂ ^(O) ₃).

Outer ring 41 and inner ring 42 serving as the race members of threepoint contact ball bearing 4 according to the present embodiment aremade of the steel having the aforementioned proper componentcomposition, and outer ring nitrogen-enriched layer 41B and inner ringnitrogen-enriched layer 42B having the nitrogen concentrations of atleast 0.05 mass % are formed on the regions including outer ring racewaysurface 41A and inner ring raceway surface 42A formed on the surfacesthereof. The totals of the carbon concentrations and the nitrogenconcentrations in outer ring nitrogen-enriched layer 41B and inner ringnitrogen-enriched layer 42B are so set in the proper range of at least0.55 mass % and not more than 1.9 mass % that sufficient hardness issupplied to surface layer portions thereof and formation of grainboundary precipitates is suppressed. Consequently, outer ring 41 andinner ring 42 serving as the race members in the present embodiment arebearing components made of steel containing at least 4 mass % ofchromium and provided with nitrogen-enriched layers on the surface layerportions thereof, while sufficiently ensuring fatigue resistance andtoughness.

In three-point contact ball bearing 4 according to the presentembodiment, balls 43 serving as the rolling elements are made ofceramics. Thus, smearing is reduced/prevented, and furthermore, as outerand inner rings 41 and 42 and balls 43 coming into contact with eachother are made of different materials, the seizure resistance isimproved. Consequently, smearing resistance, and simultaneously,endurance in an insufficiently lubricated environment, such as dry-runperformance, are improved. The ceramics having higher hardness than thesteel is employed as the material for balls 43, whereby durability ofballs 43 is improved in a contaminated environment. Further, balls 43are so made of the ceramics that reduction in hardness of balls 43 in ahigh-temperature environment is suppressed. In addition, balls 43 are somade of the ceramics that the weights of balls 43 as well as centrifugalforce acting on balls 43 are reduced as compared with a case where balls43 are made of steel, whereby three-point contact ball bearing 4 issuitable as a rolling bearing supporting a member rotating at a highspeed, in particular.

In three-point contact ball bearing 4 according to the presentembodiment, as hereinabove described, outer ring 41 and inner ring 42serving as the race members are made of the steel containing at least 4mass % of chromium, and ball 43 serving as the rolling element is madeof ceramic, whereby reduction in hardness of the bearing components in ahigh-temperature environment is suppressed. Further, outer ringnitrogen-enriched layer 41B and inner ring nitrogen-enriched layer 42Bin which the totals of the carbon concentrations and the nitrogenconcentrations are set in the proper range are formed on the regionsincluding outer ring raceway surface 41A and inner ring raceway surface42A of outer ring 41 and inner ring 42 made of the steel having theproper component composition, and ball 43 is made of ceramic, wherebythe durability of the bearing components in a contaminated environmentis improved. In addition, outer and inner rings 41 and 42 of steel incombination with ceramic ball 43 achieve improved smearing resistanceand improved dry-run performance. Consequently, three-point contact ballbearing 4 is a rolling bearing attaining not only suppression ofreduction in hardness of the bearing components in a high-temperatureenvironment, improvement in durability in a contaminated environment,improvement in smearing resistance, but also improvement of the dry-runperformance.

Referring to FIG. 15 and FIG. 16, outer ring 51, inner ring 52 androllers 53 of cylindrical roller bearing 5 in an exemplary variation ofthe present embodiment correspond to outer ring 41, inner ring 42 andballs 43 of three-point contact ball bearing 4 described above,respectively, have similar structures, and exhibit similar effects. Morespecifically, outer ring 51 and inner ring 52 serving as race membersare constituted of steel containing at least 0.11 mass % and not morethan 0.15 mass % of carbon, at least 0.1 mass % and not more than 0.25mass % of silicon, at least 0.15 mass % and not more than 0.35 mass % ofmanganese, at least 3.2 mass % and not more than 3.6 mass % of nickel,at least 4 mass % and not more than 4.25 mass % of chromium, at least 4mass % and not more than 4.5 mass % of molybdenum and at least 1.13 mass% and not more than 1.33 mass % of vanadium with a remainder consistingof iron and impurity.

Referring to FIG. 16, outer ring nitrogen-enriched layer 51B and innerring nitrogen-enriched layer 52B having nitrogen concentrations of atleast 0.05 mass % are formed on regions including outer ring racewaysurface MA and inner ring raceway surface 52A which are the surfaces ofouter ring 51 and inner ring 52. The totals of carbon concentrations andthe nitrogen concentrations in outer ring nitrogen-enriched layer 51Band inner ring nitrogen-enriched layer 52B are at least 0.55 mass % andnot more than 1.9 mass %.

Rollers 53 serving as the rolling elements are made of ceramics, such assintered body composed mainly of silicon nitride.

In the present exemplary variation, cylindrical roller bearing 5, aswell as three-point contact ball bearing 4 described above, has outerring 51 and inner ring 52 serving as the race members, made of the steelcontaining at least 4 mass % of chromium, and roller 53, serving as therolling element, made of ceramic, whereby reduction in hardness of thebearing components in a high-temperature environment is suppressed.Further, outer ring nitrogen-enriched layer 51B and inner ringnitrogen-enriched layer 52B in which the totals of the carbonconcentrations and the nitrogen concentrations are set in the properrange are formed on the regions including outer ring raceway surface 51Aand inner ring raceway surface 52A of outer ring 51 and inner ring 52made of the steel having the proper component composition, and roller 53is made of ceramic, whereby the durability of the bearing components ina contaminated environment is improved. In addition, outer and innerrings 51 and 52 of steel in combination with ceramic roller 53 achieveimproved smearing resistance and improved dry-run performance.Consequently, cylindrical roller bearing 5 is a rolling bearingattaining not only suppression of reduction in hardness of the bearingcomponents in a high-temperature environment, improvement in durabilityin a contaminated environment, improvement in smearing resistance, butalso improvement of the dry-run performance.

A method of producing a rolling bearing according to the fourthembodiment will now be described.

Referring to FIG. 20, the method of producing a rolling bearingaccording to the present embodiment includes a race member preparingstep including steps (S310) to (S410), a rolling element preparing stepincluding steps (S510) to (S550) and an assembling step carried out as astep (S600).

The race member preparing step is first described. In a steel memberpreparing step carried out as the step (S310), steel members made ofsteel containing at least 0.11 mass % and not more than 0.15 mass % ofcarbon, at least 0.1 mass % and not more than 0.25 mass % of silicon, atleast 0.15 mass % and not more than 0.35 mass % of manganese, at least3.2 mass % and not more than 3.6 mass % of nickel, at least 4 mass % andnot more than 4.25 mass % of chromium, at least 4 mass % and not morethan 4.5 mass % of molybdenum and at least 1.13 mass % and not more than1.33 mass % of vanadium with a remainder consisting of iron andimpurity, and formed into substantial shapes of race members areprepared. More specifically, working operations such as cutting, forgingand turning are performed on steel bars or steel wires containing theaforementioned components, thereby preparing steel members formed intosubstantial shapes of outer rings 41, 51 and inner rings 42, 52 as therace members.

Then, a heat treatment step performing heat treatment includingquenching and nitriding is carried out on the aforementioned steelmembers prepared in the step (S310). This heat treatment step includes acarburizing step carried out as the step (S320), a quenching stepcarried out as the step (S330), a first tempering step carried out asthe step (S340), a first subzero step carried out as the step (S350), asecond tempering step carried out as the step (S360), a second subzerostep carried out as the step (S370), a third tempering step carried outas the step (S380), a plasma nitriding step carried out as the step(S390) and a diffusion step carried out as the step (S400). This heattreatment step can be performed similarly as done in the thirdembodiment.

Then, a finishing step is carried out as the step (S410) on the steelmembers subjected to the heat treatment step. More specifically, outerring raceway surfaces 41A, 51A, inner ring raceway surfaces 42A, 52Aetc. of the steel members subjected to the heat treatment step arepolished, for example. This completes the race member in the presentembodiment and the race member preparing step in the present embodimentis completed.

According to the race member preparing step in the above embodiment,race members (outer rings 41, 51 and inner rings 42, 52 etc.) made ofsteel containing at least 4 mass % of chromium, provided with hardnitrogen-enriched layers by nitriding surface layer portions andinhibited from formation of grain boundary precipitates can be produced.Consequently, the nitrogen-enriched layers having the nitrogenconcentrations of at least 0.05 mass %, the totals of the carbonconcentrations and the nitrogen concentrations of at least 0.55 mass %and not more than 1.9 mass %, the thicknesses of at least 0.1 mm and thehardness of at least 800 HV are formed on the regions including thesurfaces (outer ring raceway surfaces 41A, 51A, inner ring racewaysurfaces 42A, 52A etc.) of the race members (outer rings 41, 51 andinner rings 42, 52 etc.) in the present embodiment as hereinabovedescribed, while the number of detected grain boundary precipitates canbe reduced to not more than one when each nitrogen-enriched layer is cutalong a cross section perpendicular to the surface thereof and the crosssection is observed with an optical microscope or an SEM randomly infive fields of view each of a square region including the surface andhaving each side of 150 μm.

On the other hand, with reference to FIG. 20, in the rolling elementpreparing step, a powder preparing step, a mixing step, a shaping step,a sintering step and a finishing step are performed as steps(S510)-(S550) sequentially. Steps (S510)-(S550) can be performedsimilarly as done in steps (S110)-(S150) in the second embodiment. Then,referring to FIG. 20, the assembling step of assembling the completedbearing components into a rolling bearing is carried out as the step(S600). More specifically, outer ring 41, inner ring 42 and balls 43prepared through the aforementioned steps and cage 44 separatelyprepared are assembled into three-point contact ball bearing 4, forexample. Thus, the rolling bearing according to the present embodimentis completed.

Note that while in the above embodiment the present mechanical componenthas been described by way of example as a mechanical componentconfiguring a deep groove ball bearing, a thrust needle roller bearing,and a constant velocity joint, the present mechanical component is notlimited thereto, and may be a mechanical component required to have asurface layer portion having fatigue resistance, wear resistance and thelike, such as a mechanical component configuring a hub, a gear, a shaftand the like. Furthermore, while in the above embodiment the presentrolling bearing has been described by way of example as a three-pointcontact ball bearing and a cylindrical roller bearing, the presentrolling bearing is not limited thereto and can be applied to a deepgroove ball bearing, an angular contact ball bearing, a thrust needleroller bearing, or other various types of rolling bearings.

Example 1

Example 1 of the present invention will now be described. A samplehaving a structure similar to that of the present mechanical componentwas actually prepared in the method of producing the mechanicalcomponent that adopts the heat treatment method for steel according tothe first embodiment, and subjected to an experiment of confirming thatformation of grain boundary precipitates on a surface layer portion wassuppressed. The procedure of the experiment is as follows:

First, a specimen having an outer diameter φ of 40 mm, an inner diameterφ of 30 mm and a thickness t of 16 mm was produced by preparing andworking a steel material made of AMS 6490 (AISI M50), a steel containingat least 0.77 mass % and not more than 0.85 mass % of carbon, at least0.01 mass % and not more than 0.25 mass % of silicon, at least 0.01 mass% and not more than 0.35 mass % of manganese, at least 0.01 mass % andnot more than 0.15 mass % of nickel, at least 3.75 mass % and not morethan 4.25 mass % of chromium, at least 4 mass % and not more than 4.5mass % of molybdenum and at least 0.9 mass % and not more than 1.1 mass% of vanadium with a remainder consisting of iron and impurity.

Then, a heat treatment step employing the heat treatment method forsteel, as described with reference to FIG. 11 in the above embodimentwas carried out on this specimen. The temperatures T₁, T₂, T₃, T₄ and T₅and the times t₁, t₂, t₃, t₄ and t₅ were so set that the hardness of thespecimen after the third tempering step was at least 58 HRC and not morethan 65 HRC, while the temperatures T₆ and T₇ were both set to 430° C.and the times t₆ and t₇ were set to 10 hours and 160 hours respectively.In the plasma nitriding step, the discharge voltage and the dischargecurrent were controlled in the ranges of at least 200 V and not morethan 450 V and at least 1 A and not more than 5 A respectively, so thatthe treatment temperature T₆ in the plasma nitriding was 430° C. In theplasma nitriding step, further, gas was introduced into a furnace in theratio of nitrogen (N₂):hydrogen (H₂)=1:1 so that the pressure in thefurnace was at least 267 Pa and not more than 400 Pa in the plasmanitriding.

The diffusion step was so carried out that the specimen was heated in anatmosphere furnace with an atmosphere of nitrogen to adjust the total ofa carbon concentration and a nitrogen concentration in the surface ofthe specimen being not more than 1.9 mass %. The specimen subjected tothe heat treatment method for steel in the present invention wasemployed as the sample according to Example of the present invention(Example A of the present invention).

On the other hand, a heat treatment step similar to the heat treatmentmethod for steel, as described with reference to FIG. 11 in the aboveembodiment was carried out on a similarly prepared specimen of AMS 6490without carrying out the diffusion step. The temperatures T₁, T_(Z), T₃,T₄ and T₅ and the times t_(i), t₂, t₃, t₄ and t₅ were so set that thehardness of the specimen after the third tempering step was at least 58HRC and not more than 65 HRC, while the temperature T₆ was set to 480°C. and the time t₆ was set to 30 hours. In the plasma nitriding step,the discharge voltage and the discharge current were controlled in theranges of at least 200 V and not more than 450 V and at least 1 A andnot more than 5 A respectively, so that the treatment temperature T₆ inthe plasma nitriding was 480° C. In the plasma nitriding step, further,gas was introduced into a furnace in the ratios of nitrogen(N₂):hydrogen (H₂):methane (CH₄)=79:80:1 so that the pressure in thefurnace was at least 267 Pa and not more than 400 Pa in the plasmanitriding. The specimen subjected to the aforementioned heat treatmentmethod was employed as a sample according to comparative example(comparative example A).

The samples according to Example A of the present invention andcomparative example A prepared in the aforementioned manner were cutalong sections perpendicular to the surfaces thereof, and these sectionswere polished. Further, the polished sections were etched with anetchant, and five fields of view of square regions of 150 μm on eachside including the surface were thereafter randomly observed on eachsample.

The results of the experiment will now be described with reference toFIG. 21 to FIG. 26. Referring to FIG. 21 and FIG. 24, upper portions ofphotographs correspond to the surfaces of the samples. Referring to FIG.22 and FIG. 25, the axes of abscissas show depths (distances) from thesurfaces, and the axes of ordinates show hardness levels (Vickershardness). Referring to FIG. 23 and FIG. 26, the axes of abscissas showthe depths (distances) from the surfaces and the axes of ordinates showthe concentrations of carbon and nitrogen, while carbon concentrations(C concentrations), nitrogen concentrations (N concentrations) andtotals (C+N concentrations) of the carbon concentrations and thenitrogen concentrations are shown in these drawings.

Referring to FIG. 21, no grain boundary precipitates (nitride of ironhaving an aspect ratio of at least 2 and a length of at least 7.5 μm)are observed on the surface layer portion of the sample according toExample A of the present invention, and the sample has an excellentmicrostructure. Referring to FIG. 22 and FIG. 23, a region of the sampleaccording to Example A of the present invention within 0.5 mm in depthfrom the surface has sufficient hardness of at least 950 HV, withpenetration of a sufficient quantity of nitrogen. When finishing such aspolishing is performed on the surface of a steel member subjected to aheat treatment similar to that in Example A of the present invention,therefore, a mechanical component provided with a nitrogen-enrichedlayer having a nitrogen concentration of at least 0.05 mass %, a totalof a carbon concentration and the nitrogen concentration of at least0.82 mass % and not more than 1.9 mass %, a thickness of at least 0.11mm and hardness of at least 830 HV can be produced so that the number ofgrain boundary precipitates is not more than one in five fields of viewof square regions of 150 μm on each side when the nitrogen-enrichedlayer is observed with a microscope.

Referring to FIG. 24, on the other hand, a large number of grainboundary precipitates 90 are observed in the surface layer portion ofthe sample according to comparative example A out of the range of thepresent invention. Referring to FIG. 25 and FIG. 26, a region of thesample according to comparative example A within 0.5 mm in depth fromthe surface has sufficient hardness of at least 950 HV with penetrationof a sufficient quantity of nitrogen, similarly to the sample accordingto Example A of the present invention. When finishing such as polishingis performed on the surface of a steel member subjected to a heattreatment similar to that in comparative example A, therefore, amechanical component having grain boundary precipitates remaining in asurface layer portion is obtained, although the surface layer portionthereof has high hardness. This mechanical component cannot be regardedas having sufficient fatigue resistance and toughness as describedabove.

Thus, it has been confirmed that a mechanical component made of steelcontaining at least 3.75 mass % of chromium and provided with anitrogen-enriched layer formed on a surface layer portion thereof whilesufficiently ensuring fatigue resistance and toughness can be producedaccording to the method of producing a mechanical component employingthe heat treatment method for steel according to the above embodiment.

Example 2

Example 2 of the present invention will now be described. An experimentof investigating the proper range of the heating temperature in thediffusion step of the heat treatment method for steel, as described inthe first embodiment, was conducted. The procedure of the experiment isas follows:

First, a specimen having an outer diameter φ of 40 mm, an inner diameterφ of 30 mm and a thickness t of 16 mm was produced by preparing andworking a steel material made of AMS 6490 (AISI M50), a steel containingat least 0.77 mass % and not more than 0.85 mass % of carbon, at least0.01 mass % and not more than 0.25 mass % of silicon, at least 0.01 mass% and not more than 0.35 mass % of manganese, at least 0.01 mass % andnot more than 0.15 mass % of nickel, at least 3.75 mass % and not morethan 4.25 mass % of chromium, at least 4 mass % and not more than 4.5mass % of molybdenum and at least 0.9 mass % and not more than 1.1 mass% of vanadium with a remainder consisting of iron and impurity.

Then, the steps from the quenching step to the third tempering stepincluded in the heat treatment step employing the method ofheat-treating steel described in the above embodiment with reference toFIG. 11 were carried out on this specimen similarly to the case ofExample A of the present invention in the aforementioned Example 1.Then, a step similar to the diffusion step was carried out bymaintaining the specimen at temperatures of 430° C. to 570° C. forvarious times, and hardness of the specimen was measured. The results ofthe measurement were analyzed on the basis of reaction kinetics, forcalculating the relation between the heat treatment time (diffusiontime) at each heating temperature in the diffusion step and thehardness.

On the other hand, another experiment was conducted by carrying out thesteps from the quenching step to the third tempering step on a similarspecimen similarly to the case of Example A of the present invention inthe aforementioned Example 1 and thereafter actually performing a plasmanitriding step and a diffusion step, for confirming a hardnessdistribution in the specimen. In the plasma nitriding step, plasmanitriding was performed by controlling a discharge voltage and adischarge current in the ranges of at least 200 V and not more than 450V and at least 1 A and not more than 5 A respectively so that thetreatment temperature T₆ in the plasma nitriding was 480° C. andmaintaining the specimen at this temperature for one hour. In the plasmanitriding step, further, gas was introduced into a furnace in the ratioof nitrogen (N₂):hydrogen (H₂)=1:1 so that the pressure in the furnacewas at least 267 Pa and not more than 400 Pa in the plasma nitriding. Inaddition, the diffusion step was carried out on the specimen completelysubjected to the plasma nitriding step by maintaining the same at 480°C. for 50 hours. A hardness distribution on a surface layer portion ofthe specimen was measured before and after the diffusion step.

The results of the experiments will now be described with reference toFIG. 27 and FIG. 28. Referring to FIG. 27, the axis of abscissas showsheat treatment times (diffusion times), and the axis of ordinates showshardness levels of the specimens. Referring to FIG. 28, the axis ofabscissas shows depths (distances) from the surfaces, and the axis ofordinates shows hardness levels. Referring to FIG. 28, rhombuses showhardness levels of the specimens not yet subjected to the diffusionsteps, and squares show hardness levels of the specimens subjected tothe diffusion steps of maintaining the same at 480° C. for 50 hours.

Referring to FIG. 27, the hardness of each specimen is reduced in ashorter time as the diffusion temperature is increased, while thereduction in the hardness is not more than 40 HV even if the diffusiontreatment is performed for 200 hours and influence exerted by thereduction in the hardness of the matrix (hardness in a region notinfluenced by penetration of nitrogen resulting from the plasmanitriding) on the hardness of the surface layer portion is reduced whenthe diffusion temperature reaches 480° C. When the diffusion temperaturereaches 460° C., the reduction in the hardness is not more than 25 HVeven if the diffusion treatment is performed for 200 hours, andinfluence exerted by the reduction in the hardness of the matrix on thehardness of the surface layer portion is further reduced. When thediffusion temperature reaches 430° C., the reduction in the hardness isnot more than 10 HV even if the diffusion treatment is performed for 200hours, and the reduction in the hardness of the matrix hardly influencesthe hardness of the surface layer portion.

Referring to FIG. 28, on the other hand, the actual reduction in thehardness of the matrix substantially coincides with the results ofanalysis shown in FIG. 27 when the diffusion step of maintaining eachspecimen at 480° C. for 50 hours is carried out, and the results ofanalysis shown in FIG. 27 conceivably coincide with the results of theactual heat treatment.

From the aforementioned results of the experiments, the heatingtemperature (diffusion temperature) in the diffusion step must be set tonot more than 480° C., and is preferably set to not more than 460° C.,in view of making nitrogen penetrating into steel reach a desired regionwhile suppressing influence exerted by reduction in the hardness of thematrix on the hardness of the surface layer portion. When the heatingtemperature is set to not more than 430° C., the diffusion step can becarried out while hardly exerting influence by reduction in the hardnessof the matrix on the hardness of the surface layer portion. While theheating temperature in the diffusion step is preferably further reducedin view of suppressing influence exerted by reduction in the hardness ofthe matrix on the hardness of the surface layer portion, this heatingtemperature is preferably set to at least 300° C., in order to preventthe time required for making nitrogen penetrating into steel reach thedesired region from being increased beyond an allowable limit in actualproduction steps.

Example 3

Example 3 of the present invention will now be described. A samplehaving a structure similar to that of the present mechanical componentwas actually prepared in the method of producing the mechanicalcomponent that adopts the heat treatment method for steel in the thirdembodiment, and subjected to an experiment of confirming that formationof grain boundary precipitates was suppressed on a surface layerportion. The procedure of the experiment is as follows:

First, a specimen having an outer diameter φ of 40 mm, an inner diameterφ of 30 mm and a thickness t of 16 mm was produced by preparing andworking a steel material made of AMS 6278 (AISI M50 NiL), a steelcontaining at least 0.11 mass % and not more than 0.15 mass % of carbon,at least 0.1 mass % and not more than 0.25 mass % of silicon, at least0.15 mass % and not more than 0.35 mass % of manganese, at least 3.2mass % and not more than 3.6 mass % of nickel, at least 4 mass % and notmore than 4.25 mass % of chromium, at least 4 mass % and not more than4.5 mass % of molybdenum and at least 1.13 mass % and not more than 1.33mass % of vanadium with a remainder consisting of iron and impurity.

Then, a heat treatment step employing the heat treatment methoddescribed with reference to FIG. 19 in the third embodiment was carriedout on this specimen. The temperatures T₁₁, T₁₂, T₁₃, T₁₄, T₁₅ and T₁₆and the times t₁₁, t₁₂, t₁₃, t₁₄, t₁₅ and t₁₆ were so set that thehardness of the specimen after the third tempering step was at least 58HRC and not more than 65 HRC, while the temperatures T₁₇ and T₁₈ wereboth set to 430° C. and the times t₁₇ and t₁₈ were set to 10 hours and160 hours respectively. In the plasma nitriding step, the dischargevoltage and the discharge current were controlled in the ranges of atleast 200 V and not more than 450 V and at least 1 A and not more than 5A respectively, so that the treatment temperature T₁₇ in the plasmanitriding was 430° C. In the plasma nitriding step, further, gas wasintroduced into a furnace in the ratio of nitrogen (N₂):hydrogen(H₂)=1:1 so that the pressure in the furnace was at least 267 Pa and notmore than 400 Pa in the plasma nitriding.

The diffusion step was so carried out that the specimen was heated in anatmosphere furnace with an atmosphere of nitrogen to adjust the total ofa carbon concentration and a nitrogen concentration in the surface ofthe specimen being not more than 1.9 mass %. The specimen subjected tothe heat treatment method for steel in the present invention asdescribed above was employed as the sample according to Example of thepresent invention (Example B of the present invention).

On the other hand, a heat treatment step similar to the heat treatmentmethod for steel, as described in the third embodiment with reference toFIG. 19, was carried out on a similarly prepared specimen of AMS 6278without carrying out the diffusion step. The temperatures T₁₁, T₁₂, T₁₃,T₁₄, T₁₅ and T₁₆ and the times t₁₁, t₁₂, t₁₃, t₁₄, t₁₅ and t₁₆ were soset that the hardness of the specimen after the third tempering step wasat least 58 HRC and not more than 65 HRC, while the temperature T₁₇ wasset to 480° C. and the time t₁₇ was set to 30 hours. In the plasmanitriding step, the discharge voltage and the discharge current werecontrolled in the ranges of at least 200 V and not more than 450 V andat least 1 A and not more than 5 A respectively, so that the treatmenttemperature T₁₇ in the plasma nitriding was 480° C. In the plasmanitriding step, further, gas was introduced into a furnace in the ratiosof nitrogen (N₂):hydrogen (H₂):methane (CH₄)=79:80:1 so that thepressure in the furnace was at least 267 Pa and not more than 400 Pa inthe plasma nitriding. The specimen subjected to the aforementioned heattreatment method was employed as a sample according to comparativeexample (comparative example B).

The samples according to Example B of the present invention andcomparative example B prepared in the aforementioned manner were cutalong sections perpendicular to the surfaces thereof, and these sectionswere polished. Further, the polished sections were etched with anetchant, and five fields of view of square regions of 150 μm on eachside including the surface were thereafter randomly observed on eachsample.

The results of the experiment will now be described with reference toFIG. 29 to FIG. 34. Referring to FIG. 29 and FIG. 32, upper portions ofphotographs correspond to the surfaces of the samples. Referring to FIG.30 and FIG. 33, the axes of abscissas show depths (distances) from thesurfaces, and the axes of ordinates show hardness levels (Vickershardness). Referring to FIG. 31 and FIG. 34, the axes of abscissas showthe depths (distances) from the surfaces, and the axes of ordinates showthe concentrations of carbon and nitrogen with thin lines and thicklines respectively.

Referring to FIG. 29, no grain boundary precipitates (nitride of ironhaving an aspect ratio of at least 2 and a length of at least 7.5 μm)are observed on the surface layer portion of the sample according toExample B of the present invention, and the sample has an excellentmicrostructure. Referring to FIG. 30 and FIG. 31, a region of the sampleaccording to Example B of the present invention within 0.5 mm in depthfrom the surface has sufficient hardness of at least 950 HV, withpenetration of a sufficient quantity of nitrogen. When finishing such aspolishing is performed on the surface of a steel member subjected to aheat treatment similar to that in Example B of the present invention,therefore, a mechanical component provided with a nitrogen-enrichedlayer having a nitrogen concentration of at least 0.05 mass %, a totalof a carbon concentration and the nitrogen concentration of at least0.55 mass % and not more than 1.9 mass %, a thickness of at least 0.11mm and hardness of at least 800 HV can be produced so that the number ofgrain boundary precipitates is not more than one in five fields of viewof square regions of 150 μm on each side when the nitrogen-enrichedlayer is observed with a microscope.

Referring to FIG. 32, on the other hand, a large number of grainboundary precipitates 90 are observed in the surface layer portion ofthe sample according to comparative example B out of the range of thepresent invention. Referring to FIG. 33 and FIG. 34, a region of thesample according to comparative example B within 0.5 mm in depth fromthe surface has sufficient hardness of at least 950 HV with penetrationof a sufficient quantity of nitrogen, similarly to the sample accordingto Example B of the present invention. When finishing such as polishingis performed on the surface of a steel member subjected to a heattreatment similar to that in comparative example B, therefore, amechanical component having grain boundary precipitates remaining in asurface layer portion is obtained although the surface layer portionthereof has high hardness. This mechanical component cannot be regardedas having sufficient fatigue resistance and toughness as describedabove.

Thus, it has been confirmed that the present mechanical component madeof steel containing at least 4 mass % of chromium and provided with anitrogen-enriched layer formed on a surface layer portion thereof whilesufficiently ensuring fatigue resistance and toughness can be producedaccording to the method of producing a mechanical component employingthe heat treatment method for steel according to the third embodiment.

Example 4

Example 4 of the present invention will now be described. An experimentof investigating the proper range of the heating temperature in thediffusion step of the heat treatment method for steel, as described withreference to the third embodiment, was conducted. The procedure of theexperiment is as follows:

First, a specimen having an outer diameter φ of 40 mm, an inner diameterφ of 30 mm and a thickness t of 16 mm was produced by preparing andworking a steel material made of AMS 6278 (AISI M50 NiL), a steelcontaining at least 0.11 mass % and not more than 0.15 mass % of carbon,at least 0.1 mass % and not more than 0.25 mass % of silicon, at least0.15 mass % and not more than 0.35 mass % of manganese, at least 3.2mass % and not more than 3.6 mass % of nickel, at least 4 mass % and notmore than 4.25 mass % of chromium, at least 4 mass % and not more than4.5 mass % of molybdenum and at least 1.13 mass % and not more than 1.33mass % of vanadium with a remainder consisting of iron and impurity.

Then, the steps from the carburizing step to the third tempering stepincluded in the heat treatment step employing the method ofheat-treating steel described in the third embodiment with reference toFIG. 19 were carried out on this specimen similarly to the case ofExample B of the present invention in the aforementioned Example 3. Astep similar to the diffusion step was carried out by maintaining thespecimen at temperatures of 430° C. to 570° C. for various times, andhardness of a carburized layer was measured. More specifically, hardnesswas measured on nine points in a region having a distance of at least0.2 mm and not more than 0.4 mm from the surface of the specimen, andthe lowest hardness was calculated. The results of the measurement wereanalyzed on the basis of reaction kinetics, for calculating the relationbetween the heat treatment time (diffusion time) at each heatingtemperature in the diffusion step and the hardness of the carburizedlayer.

On the other hand, another experiment was conducted by carrying out thesteps from the carburizing step to the third tempering step on a similarspecimen similarly to the case of Example B of the present invention inthe aforementioned Example 3 and actually performing a plasma nitridingstep and a diffusion step for confirming a hardness distribution in thespecimen. In the plasma nitriding step, plasma nitriding was performedby controlling a discharge voltage and a discharge current in the rangesof at least 200 V and not more than 450 V and at least 1 A and not morethan 5 A respectively so that the treatment temperature T₁₇ in theplasma nitriding was 480° C. and maintaining the specimen at thistemperature for one hour. In the plasma nitriding step, further, gas wasintroduced into a furnace in the ratio of nitrogen (N₂):hydrogen(H₂)=1:1 so that the pressure in the furnace was at least 267 Pa and notmore than 400 Pa in the plasma nitriding. In addition, the diffusionstep was carried out on the specimen completely subjected to the plasmanitriding step by maintaining the same at 480° C. for 50 hours. Ahardness distribution on a surface layer portion of the specimen wasmeasured before and after the diffusion step.

The results of the experiments will now be described with reference toFIG. 35 and FIG. 36. Referring to FIG. 35, the axis of abscissas showsheat treatment times (diffusion times), and the axis of ordinates showshardness levels of the carburized layer. Referring to FIG. 36, the axisof abscissas shows depths (distances) from the surfaces, and the axis ofordinates shows hardness levels. Referring to FIG. 36, rhombuses showhardness levels of the specimens not yet subjected to the diffusionsteps, and squares show hardness levels of the specimens subjected tothe diffusion steps of maintaining the same at 480° C. for 50 hours.

Referring to FIG. 35, the hardness of the carburized layer of eachspecimen is reduced in a shorter time as the diffusion temperature isincreased, while the reduction in the hardness is not more than 50 HVeven if the diffusion treatment is performed for 200 hours and influenceexerted by reduction in the hardness of the matrix (hardness in a regionof the carburized layer not influenced by penetration of nitrogenresulting from plasma nitriding) on the hardness of the surface layerportion is reduced when the diffusion temperature reaches 480° C. Whenthe diffusion temperature reaches 460° C., the reduction in the hardnessis not more than 30 HV even if the diffusion treatment is performed for200 hours, and influence exerted by the reduction in the hardness of thematrix on the hardness of the surface layer portion is further reduced.When the diffusion temperature reaches 430° C., the reduction in thehardness is not more than 10 HV even if the diffusion treatment isperformed for 200 hours, and the reduction in the hardness of the matrixhardly influences the hardness of the surface layer portion.

Referring to FIG. 36, on the other hand, the actual reduction in thehardness of the matrix substantially coincides with the results ofanalysis shown in FIG. 35 when the diffusion step of maintaining eachspecimen at 480° C. for 50 hours is carried out, and the results ofanalysis shown in FIG. 35 conceivably coincide with the results of theactual heat treatment.

From the aforementioned results of the experiments, the heatingtemperature (diffusion temperature) in the diffusion step must be set tonot more than 480° C., and is preferably set to not more than 460° C.,in view of making nitrogen penetrating into steel reach a desired regionwhile suppressing influence exerted by reduction in the hardness of thematrix on the hardness of the surface layer portion. When the heatingtemperature is set to not more than 430° C., the diffusion step can becarried out while hardly exerting influence by reduction in the hardnessof the matrix on the hardness of the surface layer portion. While theheating temperature in the diffusion step is preferably further reducedin view of suppressing influence exerted by reduction in the hardness ofthe matrix on the hardness of the surface layer portion, this heatingtemperature is preferably set to at least 300° C., in order to preventthe time required for making nitrogen penetrating into steel reach thedesired region from being increased beyond an allowable limit in actualproduction steps.

It should be understood that the embodiments and examples disclosedherein are illustrative and non-restrictive in any respect. The scope ofthe present invention is defined by the terms of the claims, rather thanthe description above, and is intended to include any modificationswithin the scope and meaning equivalent to the terms of the claims.

INDUSTRIAL APPLICABILITY

The present mechanical component is advantageously applicable tomechanical components that are formed of steel containing at least 3.75mass % of chromium and have a surface layer portion having a nitrogenenriched layer, in particular. Furthermore, the present rolling bearingis advantageously applicable to rolling bearings required to exhibitimproved durability under severe conditions, in particular.

1. A mechanical component formed of steel containing at least 0.77 mass% and not more than 0.85 mass % of carbon, at least 0.01 mass % and notmore than 0.25 mass % of silicon, at least 0.01 mass % and not more than0.35 mass % of manganese, at least 0.01 mass % and not more than 0.15mass % of nickel, at least 3.75 mass % and not more than 4.25 mass % ofchromium, at least 4 mass % and not more than 4.5 mass % of molybdenumand at least 0.9 mass % and not more than 1.1 mass % of vanadium with aremainder consisting of iron and impurity, and having a surface, saidsurface being included in a region having a nitrogen enriched layerhaving a nitrogen concentration of at least 0.05 mass %, said nitrogenenriched layer having a carbon concentration and the nitrogenconcentration, in total, of at least 0.82 mass % and not more than 1.9mass %.
 2. The mechanical component according to claim 1, wherein saidnitrogen enriched layer has a thickness of at least 0.11 mm.
 3. Themechanical component according to claim 1, wherein said nitrogenenriched layer has a hardness of at least 830 HV.
 4. The mechanicalcomponent according to claim 1, wherein said nitrogen enriched layer, asobserved with a microscope, has one or less nitride of iron having anaspect ratio of at least 2 and a length of at least 7.5 μm in fivefields of view each of a square region having each side of 150 μm. 5.The mechanical component according to claim 1, used as a componentconfiguring a bearing.
 6. A rolling bearing comprising: a race member;and a plurality of rolling elements disposed in contact with said racemember on an annular raceway, said race member being the mechanicalcomponent according to claim 1, said rolling element being formed ofceramic.
 7. The rolling bearing according to claim 6, provided in a gasturbine engine and supporting a rotary member that is one of a mainshaft and a member rotating as said main shaft rotates, relative to amember adjacent to said rotary member rotatably.